For the pastry
250g plain flour
125g unsalted butter, room temperature, cut into small cubes
A good pinch of salt
1 egg, plus one egg yolk
About 2-3 tbsps water
For the filling
About 8 thin leeks, well washed and trimmed
80ml crème fraiche
2 egg yolks
1 egg white
1 tbsp Dijon mustard
1 tsp fresh thyme leaves
100 g gruyere cheese, grated
Salt and pepper
Put the flour, butter and salt into a food processor and pulse briefly – you still want little, pea-sized pieces of butter in the mix. Drop in the egg and yolk and pulse until combined. Add the water a little at a time, and pulse until it gathers together into a ball – you may not need all of the water.
Place the dough on a floured surface and, kneed with the heel of your hands, roll into a ball, wrap in cling film and rest for at least 20 minutes.
Preheat the oven to 180C.
Once the short-cut pastry has rested, roll it on the bench top until 1cm thick and stretch over tart dish, round one of about 24cm diameter. Cover the pastry with baking paper and dried beans or rice, and blind bake the pastry for 15 minutes at 180C.
Put the finely chopped leeks in a pan with chopped thyme and olive oil, and saute. Allow to cool.
Whisk together the crème fraiche, egg yolks, mustard, thyme, cheese, ham, salt and pepper. Mix in the cooled leeks and spoon into the base. Bake for 15-20 minutes at 175C until golden. Serve while warm.