Pulse or rub the flour, salt, butter, and sugar together until the mixture resembles fine breadcrumbs. Add the egg yolk and enough ice-cold water to bring everything together into a dough without kneading.

Flatten the pastry into a disc, wrap it, and chill for at least 30 minutes. Roll it on a lightly floured surface, line a 23 cm loose-bottom tart tin, press it gently into the corners, and trim the edge.

Chill the lined tin for another 10–15 minutes if the pastry has softened. Prick the base lightly with a fork, line it with parchment, and fill with baking beans or dried beans.

Bake at 190°C conventional/170°C fan for about 15 minutes. Remove the parchment and weights, then bake for 5 minutes more, until the base looks dry but is not deeply browned.

Toss the rhubarb with the sugar, cornflour, citrus zest, and optional vanilla. Let it stand for 10–15 minutes so the cornflour can begin absorbing the fruit juices.

Rub the crumble butter into the flour and salt, then stir in the brown sugar and optional oats or chopped almonds. Keep the mixture in irregular crumbs rather than working it into a smooth dough.

Spread the rhubarb mixture over the blind-baked pastry shell and scatter the crumble evenly across the surface. For a crisper topping, chill the crumble for 10 minutes before adding it.

Place the tart on a baking sheet, reduce the oven to 180°C conventional/160°C fan, and bake for 35–45 minutes. The tart is ready when the rhubarb is tender, the juices are bubbling, and the crumble is golden brown.

Cool the tart in its tin for at least 20–30 minutes before slicing. Serve warm or at room temperature with custard, pouring cream, or vanilla ice cream.