Flamiche aux Poireaux (Belgian Leek Tart)

Combine the flour and salt in a large bowl. Add the cold cubed butter and rub it into the flour with your fingertips until the mixture resembles coarse breadcrumbs with some pea-sized pieces remaining. Mix in the egg yolk, then add the ice-cold water one tablespoon at a time, pressing the dough together until it just holds. Flatten into a disc and wrap in beeswax wrap or cling film.

Refrigerate the pastry disc for at least 1 hour.

Melt the 60 g butter in a wide, heavy-bottomed pan over a low heat. Add the sliced leeks and 1 tsp salt, stir to coat, then cover the pan with a lid.

Cook the leeks over the lowest possible heat, stirring every 10 minutes, for 40–45 minutes until completely collapsed, sweet-smelling, and beginning to turn pale golden at the edges. Remove the lid for the last 10 minutes to drive off excess moisture — the mixture should not pool liquid. Remove from heat and leave to cool to room temperature.

Preheat the oven to 190°C (170°C fan). Roll the chilled pastry out on a lightly floured surface to a 3 mm thickness and use it to line a 23 cm loose-bottomed tart tin. Trim the edges and prick the base all over with a fork.

Chill the lined tart tin in the freezer.

Line the pastry case with baking parchment, fill with baking beans or dried rice, and bake until the sides are set and pale biscuit-coloured.

Remove the baking beans and parchment and return the tart case to the oven for a further 8 minutes until the base is dry and a deep golden blonde. Remove and reduce the oven to 170°C (150°C fan).

Whisk together the double cream, 3 whole eggs, 1 egg yolk, half the Gruyère, the nutmeg, and the white pepper in a large bowl until smooth. Fold in the cooled leeks.

Pour the leek and custard mixture into the blind-baked tart case and scatter the remaining Gruyère evenly over the top.

Bake at 170°C for 28–32 minutes until the filling is just set with a very slight tremble at the very centre when the tin is nudged — the residual heat will finish it.

Allow the tart to rest before slicing and serving.