Sift flour into a mixing bowl and rub in butter. Stir in egg yolk and add enough ice-cold water to form a firm dough. Knead lightly until smooth. Cover with a piece of cling film and refrigerate for 25-30 minutes.
Roll out pastry between two plastic sheets until large enough to line a 22cm flan tin. Lift the pastry into the tin. Gently ease the pastry into the sides of the tin. Trim the edge and prick the base of the pastry with a fork. Refrigerate for 15-20 minutes.
Bake pastry, blind in a preheated oven at 180°C for about 20 minutes or until lightly golden. Cool before use.
To prepare the filling, heat butter in a saucepan and fry leek and onion until soft. Dish out and leave to cool. Spread filling in the pastry case and sprinkle with grated mozzarella cheese and add a dash of ground black pepper.
Combine whipping cream, milk and eggs with a whisk and pour the mixture over the cheese. Spread more mozzarella over it and bake at 190°C until quiche is set and golden brown.