Prepare the pastry dough – Add the plain flour to a large bowl, then rub in the cold unsalted butter using your fingertips until the mixture resembles fine breadcrumbs. Gradually add the cold water, one tablespoon at a time, and bring the mixture together into a soft dough without overworking it.

Chill the pastry – Shape the dough into a flat disc, wrap it, and place it in the fridge for 20–30 minutes. This resting time helps the pastry stay crisp and prevents shrinkage during baking.

Line the tart tin – Roll out the chilled pastry on a lightly floured surface until large enough to line a 23 cm loose-bottom tart tin. Gently press it into the base and sides, trim off excess, and prick the base lightly with a fork.

Blind bake the base – Line the pastry with baking paper and fill with baking beans or rice. Bake in a preheated oven at 190°C (170°C fan) for 15 minutes, then remove the paper and beans and bake for a further 5 minutes until the base feels dry and lightly golden.

Soften the onions – While the pastry bakes, place the sliced onions in a frying pan over low heat with a small knob of butter or a splash of oil. Cook gently for 8–10 minutes, stirring occasionally, until soft and translucent but not browned.

Prepare the custard filling – In a mixing bowl, whisk together the eggs, double cream, milk, salt, and black pepper until smooth and well combined.

Assemble the quiche – Spread the softened onions evenly over the baked pastry base, sprinkle over the grated mature cheddar cheese, then carefully pour the egg custard mixture on top.

Bake the quiche – Return the tin to the oven and bake for 30–35 minutes, or until the filling is just set with a slight wobble in the centre and the top is lightly golden.

Rest before serving – Remove from the oven and allow the quiche to stand for 10–15 minutes. This helps it firm up slightly and makes slicing easier.