Cut the pork loin into 1 cm thin slices and turn in flour. Cut the mushrooms and onions into slices or thin rings.
Heat butter in a pan and fry the floured loin slices for 2-3 minutes on each side. Pepper, salt and keep warm on a plate.
Fry the mushrooms and onions in the frying fat and season with salt and pepper. Cover the toasted bread with meat slices and spread the onion mushrooms on top. Garnish with parsley.