Season the meat with salt and pepper. Melt the butter in a large skillet and fry the meat until well browned, then remove it from the butter with a slotted spoon and set aside.
Add the onions to the skillet and fry for around 10 minutes or until golden brown.
Add the mushrooms and fry for around 3 minutes before adding the tomatoes.
Continue cooking until getting a thick sauce, then return the meat to the skillet. Cook under low fire until getting a very thick sauce that it's almost dry.
Adjust the seasonings and serve in the breads. Top with a little of the sauce and garnish with the parsley.