Heat the oven. Preheat to 190°C/375°F. Lightly butter a medium baking dish (about 20 x 30 cm or similar).

Prep the rhubarb. Toss rhubarb with sugar, cornstarch, vanilla, citrus zest and juice, and a pinch of salt. Spread evenly in the dish. The cornstarch helps thicken the juices as it bakes.

Make the crumble. In a bowl, mix flour, brown sugar, oats, cinnamon, and salt. Rub in the cold butter with your fingertips until you have clumps and sandy bits. You want a mix of small and larger crumbs for texture.

Assemble. Scatter the crumble evenly over the rhubarb. Don’t pack it down; loose crumbs bake crisper.

Bake. Bake for 35–45 minutes until the topping is golden and the fruit is bubbling at the edges. If the top browns too fast, tent loosely with foil.

Start the custard. While the crumble bakes, gently heat milk and cream in a saucepan until steaming but not boiling. Stir in vanilla and a pinch of salt. Remove from heat.

Whisk the yolks. In a bowl, whisk egg yolks, sugar, and cornstarch until smooth and pale.

Temper and thicken. Slowly pour the hot milk mixture into the yolks while whisking. Return to the saucepan and cook over low heat, stirring constantly with a spatula, until slightly thickened and silky, 3–6 minutes. Don’t let it boil.

Finish the custard. When it coats the back of a spoon, remove from heat. If you see any lumps, strain through a sieve. Keep warm.

Rest and serve. Let the crumble sit for 10 minutes so the juices settle. Spoon into bowls and pour warm custard over the top.