Rhubarb and Custard Crumble Bars

My neighbor brought over a bundle of rhubarb one Saturday morning, stalks still damp from her garden, and I had no idea what to do with them. I'd always been intimidated by rhubarb's tartness, but she promised me it was magic with custard. That afternoon, I baked these bars for the first time, and the smell alone—buttery oats, warm vanilla, and that sharp-sweet fruit—made me understand why British bakers have been pairing these flavors for generations. They disappeared before dinner.

I once made a double batch for a school bake sale, and a quiet kid who never said much came back three times to buy more. His mum told me later he'd been asking her to grow rhubarb ever since. There's something about the contrast—the bright pink fruit against pale custard and golden crumble—that just draws people in, even before they taste it.

Ingredients

• Fresh rhubarb (400 g, trimmed and chopped): Look for firm, bright stalks and never use the leaves—they're toxic. The fruit softens beautifully and holds its shape better than frozen.
• Granulated sugar (75 g for rhubarb, 60 g for custard): Balances the rhubarb's sharpness without making it cloying. Don't skimp, or the bars will pucker your mouth.
• Lemon juice (1 tbsp): Brightens the whole rhubarb layer and keeps the color vibrant instead of muddy brown.
• Cornstarch (1 tbsp for rhubarb, 2 tbsp for custard): Thickens both layers so they set cleanly and don't weep or run when you cut into them.
• All-purpose flour (250 g): Forms the tender, crumbly base and topping that holds everything together.
• Rolled oats (100 g): Adds texture and a rustic, wholesome chew to the crumble that plain flour alone can't give.
• Unsalted butter (150 g, cold and cubed): Cold butter is essential—it creates those lovely crumbly bits instead of a greasy paste.
• Light brown sugar (100 g): Brings a subtle caramel note to the crumble that pairs beautifully with the fruit.
• Salt (1/2 tsp): Just enough to make all the sweet and tart flavors sing without tasting salty.
• Whole milk (300 ml): The base of your custard. Full-fat gives the creamiest, most luxurious texture.
• Large egg yolks (2): These thicken and enrich the custard, turning it silky and golden.
• Vanilla extract (1 tsp): Use real extract if you can—it makes the custard taste homemade, not powdered.

Instructions

Prepare Your Pan:

Preheat your oven to 180°C (350°F) and line a 23cm square baking pan with parchment, leaving wings hanging over the edges. This makes lifting out the bars so much easier later.

Cook the Rhubarb:

Combine rhubarb, sugar, lemon juice, and cornstarch in a saucepan over medium heat, stirring often until the fruit softens and the mixture thickens, about 8 to 10 minutes. Set it aside to cool while you work on the base.

Make the Crumble:

Mix flour, oats, brown sugar, and salt in a large bowl, then rub in the cold butter with your fingertips or a pastry cutter until it looks like wet sand with pea-sized lumps. Don't overmix or it'll turn to dough.

Form the Base:

Press two-thirds of the crumble firmly into the bottom of your lined pan, making sure it's even and compact. Save the rest for topping.

Bake the Base:

Bake for 15 minutes until it's lightly golden and smells toasty. Remove and let it cool slightly while you make the custard.

Prepare the Custard:

Heat the milk in a saucepan until it's steaming but not boiling. In a separate bowl, whisk egg yolks, sugar, and cornstarch until smooth, then slowly pour in the hot milk, whisking constantly to avoid scrambling the eggs. Return the mixture to the pan and cook over medium heat, stirring, until it thickens enough to coat the back of a spoon, about 2 to 3 minutes, then stir in vanilla.

Layer Everything:

Spread the cooled rhubarb evenly over the baked base, then pour the custard gently over the top. Sprinkle the reserved crumble evenly across the surface.

Bake Until Golden:

Return the pan to the oven and bake for 30 minutes, until the crumble is golden and the custard is just set with a slight jiggle in the center. Don't overbake or the custard will crack.

Cool and Chill:

Let the bars cool completely in the pan, then refrigerate for at least 2 hours before lifting them out using the parchment wings and cutting into 16 squares. Cold bars slice much cleaner.

Pin it

One spring afternoon, I packed these bars into a tin and took them on a walk to the park. My friend bit into one on a bench under a blooming cherry tree and said it tasted like her grandmother's kitchen. I hadn't met her grandmother, but I knew exactly what she meant—some flavors just feel like home, even when they're new to you.

Storing and Serving

These bars keep beautifully in an airtight container in the fridge for up to three days, and honestly, they taste even better on day two when the flavors have married. Serve them cold or at cool room temperature with a dusting of powdered sugar, or go all out with a scoop of vanilla ice cream on the side. I've also served them warm with a drizzle of cream, and no one complained.

Variations and Swaps

If you want a sharper, fruitier layer, toss in a handful of fresh raspberries with the rhubarb—it adds little pops of sweetness and a gorgeous color. You can swap the oats for more flour if you prefer a smoother crumble, though I love the texture oats bring. For a gluten-free version, use a good 1:1 baking blend, but make sure your oats are certified gluten-free too.

What to Watch For

The trickiest part is getting the custard just right—too thin and it won't set, too thick and it turns gluey. Stir constantly and pull it off the heat the moment it coats your spoon. Also, don't skip chilling the bars, even if you're impatient like I was the first time. I tried cutting them warm and ended up with a delicious but messy pile instead of neat squares.

• Use cold butter and work quickly so the crumble stays light and crumbly.
• Let each layer cool slightly before adding the next to avoid sogginess.
• A sharp knife, wiped clean between cuts, gives you the prettiest bars.

Pin it

Every time I make these, I think about that first bundle of rhubarb and how a neighbor's generosity turned into one of my favorite springtime rituals. I hope these bars become a ritual in your kitchen too.

Recipe FAQs

→ Can I use frozen rhubarb instead of fresh?

Yes, frozen rhubarb works well. Thaw and drain excess liquid before cooking to prevent the filling from becoming too watery. You may need to cook it slightly longer to achieve the right consistency.

→ How do I know when the custard layer is properly set?

The custard should be just set but still slightly wobbly in the center when you remove it from the oven. It will firm up completely as it cools and chills in the refrigerator.

→ Can I make these bars ahead of time?

Absolutely. These bars are perfect for making ahead. Prepare them up to 2 days in advance and store covered in the refrigerator. They actually taste better after chilling overnight.

→ What can I substitute for rhubarb if it's not in season?

Try tart fruits like cranberries, sour cherries, or a mix of raspberries and strawberries. Adjust the sugar level based on the sweetness of your chosen fruit.

→ Why is my crumble topping not crunchy?

Make sure your butter is very cold when mixing the crumble, and don't overwork it. Also ensure the bars cool completely and chill adequately, as this helps the layers set and maintains the crumble's texture.

→ Can I freeze these bars?

Yes, these bars freeze well. Wrap individual bars tightly in plastic wrap and store in an airtight container for up to 2 months. Thaw in the refrigerator before serving.