Rhubarb and Custard Crumble Bars

My neighbor's rhubarb patch was wildly overgrown one May, and she handed me a grocery bag full of bright pink stalks with a knowing grin. I'd never baked with rhubarb before, but I wanted something that wasn't just another pie. These bars came together almost by accident—layering tangy fruit, silky custard, and buttery crumble because I couldn't decide which dessert I craved most. The kitchen smelled like spring and butter for hours.

I brought a tray of these to a garden party once, still cool from the fridge, and they disappeared before the savory platters were even touched. People kept asking if I'd bought them from a bakery, which made me laugh because I'd been scraping burnt crumble off the oven rack just an hour earlier. There's something about the pale yellow custard peeking through the golden topping that looks almost too pretty to eat, but no one ever hesitates for long.

Ingredients

• Fresh rhubarb (400 g, trimmed and chopped): Look for firm, bright stalks and avoid any that feel rubbery; the tartness is what makes these bars sing, so don't swap it for something sweeter.
• Granulated sugar (75 g for rhubarb, 60 g for custard): Just enough to tame the rhubarb's sharpness without turning it into jam; custard sugar helps thicken and sweeten the creamy layer.
• Lemon juice (1 tbsp): Brightens the rhubarb and keeps the color vivid; freshly squeezed is worth the extra minute.
• Cornstarch (1 tbsp for rhubarb, 2 tbsp for custard): This is your thickening hero; without it, you'll have a soupy mess instead of clean slices.
• All-purpose flour (250 g): Forms the sturdy base and crumble top; measure it properly or the dough won't hold together.
• Rolled oats (100 g): Adds a hearty, nutty texture to the crumble that plain flour just can't replicate.
• Unsalted butter (150 g, cold and cubed): Cold is essential for that crumbly texture; warm butter will make the dough greasy and dense.
• Light brown sugar (100 g): Gives the crumble a gentle molasses sweetness and helps it caramelize beautifully.
• Salt (1/2 tsp): Balances all the sweetness and makes every other flavor more pronounced.
• Whole milk (300 ml): The custard base; don't skimp and use skim, or you'll lose the richness.
• Large egg yolks (2): These give the custard its silky body and pale yellow color; save the whites for meringue another day.
• Vanilla extract (1 tsp): Pure vanilla is best; it rounds out the custard with warmth that artificial versions can't touch.

Instructions

Preheat and Prepare Your Pan:

Set your oven to 180°C (350°F) and line a 23cm square baking pan with parchment paper, letting the edges hang over like little handles. This makes lifting the bars out later almost foolproof.

Cook the Rhubarb:

Combine rhubarb, sugar, lemon juice, and cornstarch in a saucepan over medium heat, stirring often as the fruit softens and releases its juice, about 8 to 10 minutes. The mixture should thicken and turn glossy; set it aside to cool while you work on the base.

Make the Crumble Mix:

In a large bowl, mix flour, oats, brown sugar, and salt, then rub in the cold butter with your fingertips or a pastry cutter until it looks like wet sand with some pea-sized bits. This texture is what gives you that perfect crumbly top.

Form the Base:

Press two-thirds of the crumble firmly into the bottom of your prepared pan, packing it down with the back of a spoon or your hands until it's even and compact. Save the rest in the bowl for later.

Bake the Base:

Slide the pan into the oven for 15 minutes until the edges start to turn golden and the surface looks dry. Pull it out 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 bubbling, then whisk together egg yolks, sugar, and cornstarch in a bowl until smooth and pale. Slowly pour the hot milk into the yolk mixture, whisking constantly so the eggs don't scramble, then return everything to the pan and cook over medium heat, stirring continuously until it thickens enough to coat the back of a spoon, about 2 to 3 minutes; stir in the vanilla off the heat.

Layer the Rhubarb and Custard:

Spread the cooled rhubarb evenly over the baked base, then gently pour the custard over the top, smoothing it out with a spatula. Work carefully so the layers stay distinct and beautiful.

Add the Crumble Topping:

Sprinkle the reserved crumble mixture evenly over the custard, covering it completely but not pressing down. The loose crumbs will bake into a golden, crunchy topping.

Bake Until Golden:

Return the pan to the oven for 30 minutes, or until the crumble is deeply golden and the custard has set but still has a slight jiggle in the center. It will firm up as it cools.

Cool and Chill:

Let the bars cool completely in the pan on a wire rack, then refrigerate for at least 2 hours before using the parchment overhang to lift them out and slice into 16 squares. Clean cuts come from a cold, firm slab and a sharp knife wiped between slices.

Pin it

The first time I served these at a family gathering, my aunt—who claims she doesn't like rhubarb—ate two bars and asked for the recipe before dessert was even over. She said the custard made the rhubarb taste less like vegetables and more like a memory of summer holidays by the coast. I think about that every time I make them now, how a single layer of creamy vanilla can change someone's mind about an ingredient they've avoided for years.

Storing and Serving

These bars keep beautifully in the fridge for up to three days, covered loosely with foil or in an airtight container. I like them best served cold, straight from the fridge, when the custard is firm and the crumble still has a bit of chew. If you want to get fancy, dust them with powdered sugar just before serving or add a small scoop of vanilla ice cream on the side, though honestly they need no help at all.

Variations and Swaps

If your rhubarb is especially tart or you're feeling adventurous, toss in a handful of fresh raspberries with the rhubarb while it cooks for a deeper berry note. You can also swap half the oats for ground almonds in the crumble if you want a richer, nuttier topping. I've made these with frozen rhubarb in the dead of winter, and while the color isn't quite as vibrant, the flavor is still bright and lovely as long as you thaw and drain the fruit first.

What to Watch For

The custard should thicken quickly once the cornstarch activates, but if it stays thin, keep cooking and whisking for another minute or two. Don't let it boil hard or the eggs will curdle. The crumble topping can brown fast in the final minutes, so if it's getting too dark before the custard sets, tent the pan loosely with foil and finish baking.

• Press the base firmly, or it will crumble apart when you try to cut the bars.
• Let the rhubarb cool before layering, or the heat will start cooking the custard unevenly.
• Use the parchment overhang as handles to lift the whole slab out cleanly.

Pin it

There's something quietly satisfying about pulling a tray of these from the oven, golden and fragrant, knowing that underneath that crumble is a tart-sweet secret only you and your guests will discover. I hope they bring you as much joy as they've brought my kitchen.

Common Questions

→ 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 have a slight wobble in the center when you gently shake the pan. It will firm up further as it cools and chills in the refrigerator.

→ Can these bars be frozen for later?

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

→ Why is my crumble topping not crispy?

Ensure the butter is cold when mixing, and don't overmix the crumble. The mixture should be coarse and crumbly. Bake until golden brown, and allow to cool completely for the best texture.

→ What can I substitute for rhubarb when it's out of season?

Tart apples, plums, or a mix of berries work well as substitutes. You may need to adjust the sugar level depending on the sweetness of your chosen fruit.

→ Do I need to peel the rhubarb before using it?

No, peeling isn't necessary for most rhubarb stalks. Simply trim the ends and chop into pieces. However, if you have very thick, fibrous stalks, you may want to remove the stringy outer layer.