Cut the onion into small cubes. Clean the mushrooms and cut them into fine slices.
Heat the butter in a pan, sauté the onion and then add the mushrooms.
Sauté for a few minutes, then add finely chopped parsley and season with salt and pepper. Spread the mushrooms on the toast slices and place the cheese on top.
Bake the mushroom toast in the oven at 180 °C for about 5 minutes.