Lay out 2 medallions of veal and place 1 slice prosciutto and 1 slice fontina cheese on top of each piece of veal. Place a piece of veal on top of each to create a sandwiched effect and dust the outsides with flour. In a large saute pan, heat olive oil on medium heat, add veal and brown on both sides, about 3 to 4 minutes per side. Remove excess oil, add mushrooms, Marsala wine and salt and pepper, to taste. Reduce for 4 to 5 minutes. Remove veal to plates and spoon mushrooms and sauce over veal.