Cut the veal into serving pieces. Pound both sides of the meat with the flat side of a meat mallet until it is 3 mm thick. Sprinkle with salt and pepper. Melt the butter over medium heat in a large frying pan. Add two of the veal pieces and cook for 1–2 minutes on each side, or until tender. Transfer the veal to a platter; cover with foil and keep warm. Repeat with the remaining veal pieces.
Add the mushrooms and onion to the pan. Cook over medium heat, stirring frequently, for 4 minutes, or until tender. Stir in the stock, wine, lemon juice and rosemary, stirring and scraping any browned residue off the bottom of the pan. Bring to the boil and cook, uncovered, for 5 minutes, or until slightly thickened. Stir in the parsley and pour the sauce over the veal. Serve immediately.
Scaloppine with Lemon Sauce:
Dust the flattened and seasoned veal lightly with flour before cooking in the butter, as described. When cooked, set aside on a platter, cover with foil and keep warm. Add 1/3 cup lemon juice and 2 tablespoons chopped parsley to the pan, remove from the heat and swirl in an extra tablespoon of butter bit by bit. Pour over the veal and serve immediately.