Heat the oven to 220°C. Fry the mushrooms in the coconut oil, stirring often, until golden brown, about 5-8 minutes. You may have to do this in batches.
Using your hands, thoroughly combine the mushrooms with the beef, onion, garlic, egg, Parmesan, salt and pepper.
Line a baking tray with parchment paper, or use a large round pizza stone.
Flatten the meat and mushroom mixture onto the paper, about 1cm thick. Bake for 10-15 minutes.
Add the toppings of your choice and return to the oven for 5 minutes, until the cheese is melted and golden. Cut into slices and serve with a salad.