For the mushroom sauce, peel and chop the onion.
Brush and clean the mushrooms and cut them into slices.
Wash the parsley, pluck off the leaves and chop.
Heat the butter in a frying pan and brown the onion.
Add the mushrooms and sauté.
Pour vegetable soup over the mushroom sauce and saute until most of the liquid has evaporated.
Sprinkle with parsley before serving.