Heat the oil in a pan and add the chopped leek. Sauté over low heat until golden.
Add the chopped mushrooms and sauté for a few minutes.
Meanwhile, in a bowl, mix the eggs, cream, salt, pepper, and nutmeg. Set aside.
Lightly roll out the pastry with a rolling pin and place it into tartlet molds or a muffin tin. Prick the base of the dough with a fork and brush it with egg. Bake for about 10 minutes at 180°C in a preheated oven, making sure it does not burn.
Once the mini quiches are golden brown, remove them from the oven.
Add the mushrooms to the egg and cream mixture, and stir in the Truffle Sauce to taste. Fill the tartlets and top with the cheese for gratinating. Bake for a few minutes until they begin to turn golden, then remove from the oven.
