Preheat the oven to 240°C/475°F/Gas mark 9. Lightly grease a non-stick baking tray.
Gently wipe the mushrooms with damp kitchen paper. Set aside.
Combine the cheese with the breadcrumbs. Add the garlic salt and black pepper.
Roll the mushrooms in the cheese mixture, and set on the greased baking tray.
Bake in the oven for 8–10 minutes until browned. Serve hot or warm.