Preheat oven to 200 C.
Wash the leeks well and finely chop. Gently saute the leeks in butter in a frying pan over a low heat. Cook until the the leeks are completely tender and then set aside to cool.
Whisk together the cream and eggs.
Roll the pastry on a lightly floured bench until they are approximately 1 1/2 times their original size.
Cut out circles with a glass or cookie cutter and place in greased patty pans.
Place a spoonful of leek in each pastry case and sprinkle with gruyere. Pour over egg and cream filling and sprinkle with a little salt and freshly ground black pepper.
Bake for 15 minutes until puffed and golden.