1) Preheat the oven to 200C/Gas 6.
2) Unroll the puff pastry on a flat surface. Roll the pastry into a 36-cm square. Using a pizza cutter or a knife, cut the pastry into twelve squares.
3) Top each square with the mushrooms (about 1 heaped teaspoon each) and 1 teaspoon of grated cheese. Wet two edges of each square with wet fingers and then fold over to make a triangle. Press the edges together with a fork.
4) Place the turnovers on a large baking sheet, about 10cm apart. Bake for 10 to 12 minutes, until puffed up and golden brown.