Peel the onion and chop finely. Cut washed peppers into small cubes. Clean mushrooms and cut into 1/2 cm thick slices. Sauté onion and pepper in butter, add and fry mushrooms. Season with salt and pepper. Drain in a colander and let cool.
Mix well cream cheese and egg yolk, stir in the mushrooms and season with salt and pepper.
Cut breadrolls into equal slices and spread the mushroom mixture on each. Preheat oven grill in position and bake the bread in the lowest level light brown.