Quarter the mushrooms and cut the onion into cubes. Cut the leek, carrots and sausages into slices.
Melt the butter in a large saucepan and fry the sausage slices on both sides. Then add the mushrooms and fry while stirring vigorously (so that they do not turn brown). If you want to keep the sausages crisp, you can take them out and sprinkle them again at the end.