Wipe the mushrooms, trim any stalks level with the caps and slice the mushrooms thinly. Peel the garlic cloves and, using the blade of a knife, mash the garlic to a purée with a little salt. Beat the garlic into the butter.
Melt the garlic butter in a frying pan and when hot add the mushrooms. Fry gently for 2–3 minutes, tossing the mushrooms in the butter. Sprinkle with extra salt to t