Remove stalks from mushrooms (don’t throw away these can be sauteed for another use), carefully remove gills with a spoon.
Spray mushrooms with cooking oil spray and place on a baking sheet lined with parchment (you want the remove gill side facing down)
Bake for 10 mins.
While baking, fry the onion and garlic in a frying pan with some cooking oil spray over a medium high heat until softened, add the passata, stock, tomato paste, oregano and parsley and simmer to reduce down to a thicker pizza sauce.
When the mushrooms have baked for 10 mins. Turn over on the baking sheet, add the pizza sauce, mozzarella and choice of toppings and place bake in the oven and back for an about 15-20 minutes until the cheese is melted and is golden.