How to cook
Wash and finely chop the leeks and mushrooms. Preheat a pan on the stove with a few tablespoons oil and fry the leeks first, then add the crushed garlic and mushrooms, stir well and add the thyme and balsamic vinegar.
Leave to cook until ready to evaporate their water. Then take them off from the heat, allow to cool and mix- in the grated cheese. Cut the puff pastry sheets into squares or rectangles. Distributed stuffing along their length, close by pinching the edges. On the surface of them all, make slight parallel incisions with a knife.
Coat them with the egg, beaten with a little oil and put them to bake in moderately hot oven.