Rhubarb and Custard Crumble Bars

My neighbor showed up one April morning with a bundle of rhubarb stalks from her garden, challenging me to make something other than pie. I'd been craving those old-fashioned rhubarb and custard sweets from the corner shop, so I thought: why not turn that into a traybake? The kitchen smelled like spring and butter for hours, and by the time they cooled, half the pan had already disappeared.

I brought a batch to a spring potluck once, and someone asked if I'd bought them from a fancy bakery. That might be the best compliment I've ever received. Watching people go back for seconds, I realized these bars have a way of making any gathering feel a little more special, even if it's just tea on a Tuesday.

Ingredients

• Fresh rhubarb (400 g): The star of the show, its tartness balances all that buttery sweetness. Always trim off the leaves, they're toxic, and chop the stalks into roughly equal pieces so they cook evenly.
• Granulated sugar (75 g for rhubarb, 60 g for custard): Just enough to tame the rhubarb without losing its edge. I've tried cutting it back, but the fruit needs this amount to shine.
• Lemon juice (1 tbsp): Brightens the rhubarb and keeps the color vivid instead of muddy brown.
• Cornstarch (1 tbsp for rhubarb, 2 tbsp for custard): This is what turns runny fruit into a thick, spoonable layer and gives the custard its body. Don't skip it or you'll have soup.
• All-purpose flour (250 g): Forms the sturdy base and crumbly topping. I use plain flour and it works perfectly every time.
• Rolled oats (100 g): Adds texture and a bit of wholesome chew to the crumble. The oats toast beautifully in the oven.
• Unsalted butter (150 g, cold and cubed): Cold butter is key, it creates those crumbly pockets instead of turning the mixture greasy.
• Light brown sugar (100 g): Gives the crumble a deeper, caramel-like sweetness that white sugar just can't match.
• Salt (1/2 tsp): A tiny amount that makes all the other flavors pop, especially the butter.
• Whole milk (300 ml): The base of your custard. Whole milk gives the creamiest result, but semi-skimmed works in a pinch.
• Egg yolks (2 large): These make the custard rich and silky. Save the whites for meringues or an omelet the next morning.
• Vanilla extract (1 tsp): Pure vanilla is worth it here, it adds warmth and rounds out the custard beautifully.

Instructions

Prep your pan:

Preheat your oven to 180°C (350°F) and line a 23cm square baking pan with parchment paper, leaving some overhang on the sides. This little paper handle will save you so much frustration when it's time to lift the bars out.

Cook the rhubarb:

Combine the rhubarb, 75 g sugar, lemon juice, and 1 tbsp cornstarch in a saucepan over medium heat. Stir often and watch it transform into a thick, glossy compote in about 8 to 10 minutes, then set it aside to cool.

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 rough breadcrumbs. It's oddly satisfying and reminds me why I love baking by hand.

Form the base:

Press two-thirds of the crumble mixture firmly into the bottom of your lined pan, packing it down with the back of a spoon or your knuckles. Reserve the rest for later.

Par-bake the base:

Bake the base 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 separate bowl, whisk together the egg yolks, 60 g sugar, and 2 tbsp cornstarch until smooth, then slowly pour in the hot milk while whisking constantly to avoid scrambling the eggs. Return everything to the pan and cook over medium heat, stirring constantly, until it thickens like pudding (about 2 to 3 minutes). Stir in the vanilla off the heat.

Layer it up:

Spread the cooled rhubarb evenly over the par-baked base, then gently pour the warm custard over the fruit. It will settle and find its level, so don't fuss too much.

Top with crumble:

Scatter the reserved crumble mixture over the custard in an even layer. It doesn't have to be perfect, a rustic look is part of the charm.

Bake until golden:

Return the pan to the oven and bake for 30 minutes, or until the crumble is deeply golden and the custard has just set with a slight wobble in the center.

Chill and slice:

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

Save

One rainy Saturday, I made these with my niece, who was convinced she hated rhubarb. She ate two bars standing at the counter, custard on her chin, and declared them magic. Sometimes the right combination changes everything, and watching her discover that was better than any compliment.

Storing and Serving

These bars keep beautifully in an airtight container in the fridge for up to three days, though they rarely last that long in my house. I like them cold straight from the fridge, but you can let them sit at room temperature for 10 minutes if you prefer a softer crumble. A light dusting of powdered sugar just before serving makes them look bakery-perfect, and a dollop of whipped cream or vanilla ice cream turns them into a proper dessert.

Customizing Your Bars

If you want a sharper, fruitier punch, fold a handful of fresh raspberries into the rhubarb as it cooks. I've also stirred in a pinch of ground ginger or cardamom to the crumble mixture for a warm, spiced note that works beautifully with the custard. Some people like to add a handful of chopped almonds or pecans to the topping for extra crunch, and I won't argue with that.

What to Do if Things Go Wrong

If your custard seems too thin, don't panic, it will firm up as it chills. If it's lumpy, you can press it through a fine sieve before pouring it over the rhubarb. If the crumble topping browns too quickly, tent the pan loosely with foil for the last 10 minutes of baking.

• Rhubarb too watery: Cook it a bit longer to evaporate excess liquid before layering.
• Base too crumbly: Next time, press it down harder and add a tablespoon of cold water to the mixture if it feels too dry.
• Custard split: Whisk in a splash of cold milk off the heat to bring it back together.

Save

There's something quietly wonderful about pulling a pan of these from the oven and knowing you've made something that tastes like childhood and springtime all at once. I hope they bring as much joy to your kitchen as they have to mine.

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 bars from becoming too watery. You may need to cook it slightly longer to achieve the proper thickness.

→ Why do the bars need to chill before cutting?

Chilling allows the custard layer to set completely, making it easier to cut clean, neat bars without the filling oozing out. At least 2 hours in the refrigerator is recommended.

→ Can I make these bars ahead of time?

Absolutely. These bars are ideal for advance preparation. Make them up to 2 days ahead and store covered in the refrigerator until ready to serve.

→ What can I substitute for rhubarb?

Try strawberries, raspberries, or a combination of berries. Adjust sugar levels based on the sweetness of your chosen fruit. Apples or plums also work well.

→ How do I know when the custard is properly cooked?

The custard should coat the back of a spoon and hold a line when you draw your finger through it. It will thicken further as it cools and sets during baking.

→ Can I freeze these bars?

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