Clean the mushrooms. Cut the earthy parts into small pieces and then grate the mushrooms with a wet dish. Then halve or quarter the mushrooms according to size.
Peel and chop half the onion. Rinse the parsley, dry it and chop it as well. Heat a coated frying pan without fat at the beginning and fry the mushrooms in it. Once a little liquid has evaporated, add the olive oil and the onion. Roast everything together over medium heat for 2 minutes.
Season the mushrooms with parsley, salt and pepper and toss them in the butter just before serving.