Heat olive oil in a 12-inch heavy skillet. While oil is heating, dredge veal cutlets in flour, 2 at a time, shaking off excess flour. Quickly saute the veal until just cooked through, 1 to 1 1/2 minutes on each side. Transfer to a shallow baking pan and keep warm, loosely covered with foil.
Deglaze pan with Marsala and chicken stock and let reduce slightly. Top each cutlet with 3 sage leaves, 2 slices prosciutto, and 4 slices Fontina. Pour the pan sauce over cutlets and bake in oven, covered, for about 10 minutes.