Preheat the oven to 200°C, Gas Mark 6. Parboil the potatoes for 5 minutes and drain. While the potatoes are boiling, heat the oil in a medium saucepan and gently sweat the leeks and mushrooms for 4-5 minutes until beginning to soften.
Stir through the creme fraiche and grated cheese, and add the parboiled potatoes. Season with a small pinch of salt and some black pepper.
Place the filling in a medium rectangular baking dish just a little smaller than the pastry sheet, and top with the sheet of puff pastry, cutting away any excess and pressing down the edges. To really impress, make a sweet decoration from the excess pastry and simply stick on with some beaten egg.
Brush all over with beaten egg and cut a hole in the top to allow steam to escape. Bake in the oven for 25-30 minutes or until golden and bubbling.