Blog

These Rhubarb Crumb Bars are what spring baking is all about: a golden, buttery oat crust pressed into a pan, a jammy-tart rhubarb filling that holds its shape when sliced, and a loose crumble topping that shatters with the first bite. You do not need a stand mixer, a candy thermometer, or any special skills. What you do need is one important pro tip: pre-bake your crust for 10 minutes before adding the filling. That single step prevents the dreaded soggy bottom and is the difference between bars that hold and bars that fall apart.​

Why This Recipe Works

Three things make this recipe reliable every single time.

First, maceration does the thickening for you. Tossing raw rhubarb with sugar and letting it sit for 30–45 minutes draws out excess liquid before baking, so you control the moisture, not the oven. You end up with a filling that is jammy but never watery.​

Second, cold butter in the crumble creates distinct, sandy crumbs. When butter is too warm, the sugars melt into it and you get a greasy, flat layer instead of crunchable clusters. Keep your butter cold right up until mixing.​

Third, baking soda in the crust adds subtle lift. Without it, the base bakes dense and cardboard-flat. A small amount creates just enough open crumb to make each bite tender rather than hard.​

Ingredients

For the Oat Crust and Crumble Topping (same dough, split in two)

For the Rhubarb Filling

Pro tip: Use stalks that are deep red for a prettier pink filling. If your rhubarb is pale green, the bars will taste identical but the filling will look more olive-coloured.

Equipment

Required: 8×8-inch (20×20 cm) square baking pan, parchment paper, kitchen scale, pastry cutter or fingertips, medium saucepan, oven or OTG at 175–180°C (350°F)

Nice-to-have: Bench scraper for pressing crust evenly, offset spatula, sieve for dusting powdered sugar on top

Step-by-Step Instructions

Step 1 — Macerate the Rhubarb (35 min, hands-off)

Slice rhubarb into 1 cm (½ inch) pieces and toss with all the sugar, lemon zest, cornstarch, vanilla, and salt in a bowl. Leave uncovered at room temperature for 30–40 minutes. Visual cue: You should see a pool of pink-red liquid collecting at the bottom. If the bowl is dry after 30 min, your rhubarb may be old — add 1 tbsp water.​

Step 2 — Make the Crust/Crumble Dough (10 min)

Preheat your oven to 175°C / 350°F. Line your pan with parchment, leaving a 2-inch overhang on two sides. In a large bowl, whisk together flour, oats, brown sugar, baking soda, cinnamon, and salt. Drop in cold cubed butter and work it in with your fingertips until the mixture looks like coarse, damp sand — some pieces as big as a pea are fine. The dough should clump when you squeeze it in your fist. Visual cue: It should look like wet beach sand, not smooth paste. If it feels greasy, your butter was too warm — refrigerate the bowl for 10 minutes.

Step 3 — Press and Pre-Bake the Crust (10 min)

Scoop out two-thirds of the crumble mixture (about 380 g / 13.5 oz) and press it firmly into the bottom of your lined pan. Use the bottom of a flat glass or measuring cup to get an even, compact layer. Bake for 10 minutes at 175°C / 350°F until the edges are just beginning to turn golden. This step is non-negotiable for a non-soggy base.​

Step 4 — Assemble and Bake (25–30 min)

Remove the pre-baked crust from the oven. Spoon the macerated rhubarb filling evenly over the hot crust — including all the accumulated juice. Scatter the remaining crumble loosely over the top. Do not press it down; keep it in rough, chunky pieces. Bake at 175°C / 350°F for 25–30 minutes, until the topping is deep golden and the filling is bubbling at the edges. Visual cue: The filling should bubble actively at the sides but not overflow. The top crumble should be the colour of a digestive biscuit.​

Step 5 — Cool Completely (minimum 1–2 hours)

This is where patience pays off. The filling sets as it cools. Cut bars before they’re fully cool and they will crumble and smear. Cool in the pan on a wire rack for at least 2 hours, or refrigerate for 1 hour if you’re impatient. Lift out using the parchment overhang and slice into 9 or 12 bars with a sharp knife.​

Troubleshooting

Substitutions & Variations

Dairy-Free: Replace butter with solid coconut oil (same weight). The crumble will be slightly more fragile but still delicious. Avoid vegan margarine spreads — too much water content makes the crumble mushy.​

Egg-Free: This recipe is already egg-free in both crust and filling. No substitution needed.

Gluten-Free: Use a 1:1 GF all-purpose flour blend (same weight) and certified GF rolled oats. Add ½ tsp xanthan gum if your blend doesn’t include it. Expect a slightly more crumbly crust — handle bars gently.

Strawberry-Rhubarb: Replace 150 g rhubarb with 150 g hulled, halved strawberries. Reduce sugar by 20 g since strawberries are sweeter.​

Spiced Brown Butter Version: Brown the butter before using it in the crumble for a nutty, caramel depth. Spread browned butter on a plate, refrigerate until solid (about 30 min), then cut into cubes and proceed as normal.

Storage, Make-Ahead & Freezing

• Counter: Covered at room temperature for up to 2 days
• Fridge: Airtight container for up to 5 days; bring to room temperature 20 minutes before serving for the best texture
• Freezer: Individually wrap bars in cling film, then place in a zip-lock bag; freeze for up to 3 months. Thaw overnight in the fridge. To re-crisp the topping, place on a baking tray in a 160°C / 325°F oven for 8–10 minutes
• Make-ahead: You can make the crumble dough up to 3 days ahead and refrigerate it. Macerate the rhubarb the morning of baking
• Do not freeze unbaked assembled bars — the filling releases too much liquid on thawing and turns the crust to mush​

Serving Ideas

Serve slightly warm with a scoop of vanilla ice cream for a more indulgent dessert. For everyday snacking, they are great at room temperature. A light dusting of powdered sugar just before serving makes them look bakery-finished. For a grown-up pairing, a dollop of barely sweetened crème fraîche cuts the tartness beautifully.

Notes from My Kitchen

Batch 1: Used warm softened butter — the topping baked flat and greasy with no crumble texture at all. Complete fail, but lesson learned.​

Batch 2: Switched to cold cubed butter and the crumble was perfect — chunky and golden. But skipped the pre-bake and the bottom was soggy in the centre.

Batch 3: Added the 10-minute pre-bake. The base was fully set, golden, and held the filling without soaking through. This became the keeper method.

Batch 4: Tested draining the macerated juice vs. using all of it. Using all of it (with enough cornstarch) gave a juicier, more jammy filling. Draining made the filling taste concentrated but slightly dry. Keep the juice.

Batch 5: Tried the strawberry-rhubarb version. Reduced sugar by 20 g and the bars were beautiful — sweet enough without tipping into candy territory.

Final method chosen: Cold butter crumble, 10-minute pre-bake, full rhubarb juice retained, 175°C / 350°F for 28 minutes. Perfect balance of tart filling and crunchy top every single time.

FAQs

Can I use frozen rhubarb?
Yes — thaw completely and drain off at least half the liquid before macerating. Frozen rhubarb releases significantly more juice. If you add it directly without draining, your filling will be watery regardless of how much cornstarch you use.

My bars are still gooey in the middle — are they underbaked?
If the top crumble is golden and the edges are actively bubbling, they are likely done. The filling will appear very soft at 175°C because cornstarch doesn’t fully set until cooled. Trust the cool time, not just the visual of the filling in the oven.​

Can I double this recipe?
Yes — use a 9×13-inch (23×33 cm) pan, keeping the same temperatures and extending the bake to 35–40 minutes. Check at 30 minutes for the crumble colour.

Best sugar for the crumble?
Light brown sugar is preferred over white because its molasses content caramelises at a lower temperature, creating deeper flavour and better colour. Dark brown sugar works too but can make the topping slightly sticky.​

Can I bake this in an OTG?
Absolutely. Preheat for a full 20 minutes. Use the middle rack. If your OTG runs hot on top, loosely tent the pan with foil for the first 15 minutes of the final bake, then remove to brown the crumble.

Can I reduce the sugar in the filling?
You can reduce it by up to 30 g (2 tbsp) without affecting structure, but rhubarb is extremely tart on its own — anything less will be quite sharp. Taste your rhubarb raw first to judge.

Wrapping It Up

Rhubarb Crumb Bars are one of those recipes that feel far more impressive than the effort required. Once you understand the two non-negotiables — cold butter for the crumble and a pre-baked crust — the rest is just layering and waiting. The waiting is the hardest part. They keep beautifully in the fridge all week, make brilliant take-to-a-friend treats, and freeze like a dream for future-you. Make them once and they will become a seasonal ritual every rhubarb season. Happy baking! 🍓