Heat oil or butter in a pan add sliced onion and cook on slow flame until soft and little caramelised. Add salt and pepper according to taste.

Take the stems of the mushrooms out, brush the mushroom with little butter, Grill or pan fry them on a medium heat until they get a bit of color on them. Not to long

Remove mushrooms from heat and stuff them with the caramelised onions and mozzarella as desired. Cover the mushroom with puff pastry

Bake them at 190 until the puff pastry is flaky and mushroom are cooked