Rhubarb and Custard Crumble Bars

My neighbor brought me armfuls of rhubarb one April morning, and I stood in the kitchen wondering what to do with all those tart pink stalks. I'd been craving something that wasn't just a crumble or a pie, something I could pack up and share easily. That's when I remembered the custard bars my grandmother used to make, and I thought: why not marry the two? The result was these layered beauties, tangy and creamy and crumbly all at once, and they disappeared from the tin before they'd even fully cooled.

I made these for a spring garden party, and people kept asking if I'd ordered them from a bakery. One friend admitted she'd never liked rhubarb until she tried these bars, because the custard mellowed the sharpness just enough. I watched her take three, and I didn't even mind.

Ingredients

• Fresh rhubarb: Use bright, firm stalks and trim away any leaves, which are toxic. The tartness is what makes these bars sing, so don't swap it for something sweeter.
• Granulated sugar (for rhubarb): This tames the rhubarb's bite and helps it break down into a thick, jammy layer.
• Lemon juice: A splash brightens the rhubarb and keeps the flavor from feeling flat or one-note.
• Cornstarch (for rhubarb): This thickens the fruit so it doesn't make the base soggy.
• All-purpose flour: The backbone of both the base and the crumble topping.
• Rolled oats: They add texture and a lovely chew to the crumble. Use old-fashioned oats, not instant.
• Unsalted butter: Cold butter is key. It should be cubed and worked in quickly so the crumble stays crumbly.
• Light brown sugar: Adds a hint of caramel warmth to the crumble layers.
• Salt: Just a pinch balances all the sweetness and makes the flavors pop.
• Whole milk: Full-fat milk gives the custard a rich, creamy texture. Don't use skim.
• Egg yolks: These thicken the custard and give it that classic golden color.
• Granulated sugar (for custard): Sweetens and helps the custard set without turning grainy.
• Cornstarch (for custard): Ensures the custard firms up enough to slice neatly once chilled.
• Vanilla extract: A teaspoon of good vanilla makes the custard taste homemade and warm.

Instructions

Prep your pan:

Preheat your oven to 180°C (350°F) and line a 23cm square baking pan with parchment, letting the edges hang over so you can lift the bars out later. This little step saves so much frustration when it's time to slice.

Cook the rhubarb:

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

Make the crumble:

In a large bowl, mix the flour, oats, brown sugar, and salt, then rub in the cold butter with your fingertips until it looks like coarse breadcrumbs. The butter should stay in little pebbles, not melt into a paste.

Form the base:

Press two-thirds of the crumble mixture firmly into the bottom of your prepared pan to create an even layer. Save the rest for the topping.

Bake the base:

Pop it in the oven for 15 minutes until it's just turning golden at the edges. 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 bowl, whisk together the egg yolks, sugar, and cornstarch until smooth, then slowly pour in the hot milk, whisking constantly so the eggs don't scramble. Return the mixture to the pan and cook over medium heat, stirring, until it thickens (2 to 3 minutes), then stir in the vanilla.

Layer it up:

Spread the cooled rhubarb evenly over the baked base, then pour the warm custard over the rhubarb in a smooth layer. Work gently so you don't disturb the fruit underneath.

Add the crumble topping:

Sprinkle the reserved crumble mixture evenly over the custard layer. Don't press it down, just let it sit on top.

Bake until golden:

Bake for 30 minutes, or until the crumble is golden brown and the custard is just set with a slight wobble in the center. It will firm up as it cools.

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 overhang and slicing into squares. Cold bars slice the cleanest.

Save

These bars became my go-to whenever I wanted to impress without the fuss of a fancy cake. I've served them at birthday teas, after Sunday roasts, and once at a bake sale where they sold out in minutes. There's something about that combination of textures and flavors that just feels like spring on a plate.

Storing and Serving

I keep these bars in an airtight container in the fridge for up to three days, though they rarely last that long. They're lovely served cold, but if you prefer, let them sit at room temperature for 10 minutes before eating so the flavors open up. A light dusting of powdered sugar or a dollop of whipped cream turns them into something truly special.

Variations and Swaps

If you want a sharper, fruitier edge, stir a handful of fresh raspberries into the rhubarb while it cooks. I've also swapped half the rhubarb for strawberries when the seasons overlap, and it's just as delicious. For a nutty twist, replace a quarter of the oats with finely chopped almonds in the crumble.

What to Watch For

The custard should be thick enough to coat the back of a spoon before you pour it over the rhubarb. If it's too thin, it won't set properly once baked. Make sure your butter is truly cold when you make the crumble, warm butter will turn the mixture greasy instead of crumbly. And don't overbake, the custard will continue to set as it cools, so a slight jiggle in the center is exactly what you want.

• Let the rhubarb cool before layering, or it might curdle the custard.
• Use parchment paper with enough overhang to lift the whole slab out easily.
• Chill fully before slicing for clean, bakery-perfect edges.

Save

I hope these bars bring you as much joy as they've brought me. There's something wonderfully satisfying about pulling a tray of these from the oven and knowing you've made something both comforting and impressive.

Recipe FAQs

→ Can I use frozen rhubarb instead of fresh?

Yes, frozen rhubarb works well. Thaw and drain excess liquid before cooking to prevent a watery filling. 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 appear slightly jiggly in the center when you gently shake the pan, but not liquid. It will firm up completely as it cools and chills in the refrigerator.

→ Can these bars be made ahead of time?

Absolutely. These bars are ideal for making ahead as they need at least 2 hours to chill. They keep well refrigerated for up to 3 days and actually taste better once fully set.

→ Why is my crumble topping not crunchy?

Make sure your butter is cold when mixing the crumble, and don't overwork the mixture. The crumble should be coarse and crumbly, not paste-like. Baking until golden ensures a crisp texture.

→ Can I substitute the rhubarb with other fruits?

Yes, you can use strawberries, raspberries, or a mix of berries. Adjust the sugar based on the sweetness of your chosen fruit, as rhubarb is particularly tart.

→ Do I need to blind bake the base?

The initial 15-minute bake of the base helps create a firmer foundation that won't become soggy from the fruit and custard layers. This step is important for the best texture.