Roll out the puff pastry into a rough circle about 25cm in diameter and prick the surface with a fork. Preheat the oven to gas 5, 170℃, fan 190℃.
Put the pastry on a baking sheet and bake in the preheated oven for about 15 minutes until golden brown. Remove and leave to cool.
Clean and trim the mushrooms. Brush off any dirt and wipe them clean, or if they are very dirty, wash them in cold water, drain and dry. Cut the mushrooms into thick strips.
Heat the olive oil in a pan and sauté the mushrooms with the crushed garlic. Season and add flatleaf parsley.
Spoon the mushrooms onto the puff pastry and sprinkle over the Parmesan. Cut into slices like a pizza and serve immediately.