Step 1 — Preheat the oven

Preheat your oven to 190°C (375°F / Gas Mark 5). Lightly grease a medium baking dish approximately 22 × 15 cm.

Step 2 — Make the lemon filling

Combine the lemon juice, lemon zest, caster sugar, cornstarch, and salt in a medium saucepan over low-medium heat. Stir continuously for 4–5 minutes until the sugar dissolves and the mixture thickens slightly. Remove from heat, add the butter cubes, and stir until fully melted. Whisk in the egg yolks one at a time, then stir in the double cream and vanilla extract. Pour the filling into your prepared baking dish and set aside.

Step 3 — Make the crumble topping

Rub the cold butter into the sifted flour using your fingertips until the mixture resembles rough, damp sand with irregular chunky clusters. Stir in the demerara sugar, ground almonds, oats, lemon zest, cinnamon, and salt until just combined. Do not overwork the mixture.

Step 4 — Assemble

Scatter the crumble topping loosely and evenly over the lemon filling. Do not press it down — keep it airy and rough for maximum crunch.

Step 5 — Bake

Bake for 30–35 minutes until the topping is deep golden-amber and the filling is bubbling gently around the edges.

Step 6 — Rest and serve

Allow the crumble to rest for at least 10 minutes before serving. Serve warm with vanilla ice cream, clotted cream, or crème fraîche.