Cut each schnitzel in half. Place a sage leaf on each one, wrap with the prosciutto, then press another sage leaf on top and flatten out with your hand. Dust lightly with the seasoned flour.
Heat the oil and the butter in a large frying pan until foaming. Fry the veal for 4-5 mins on each side until the prosciutto is crisp, then remove from the pan. Add the Marsala to the pan and sizzle to make a sauce, scraping any tasty bits off the bottom. Add the lemon juice to taste. Put the veal back in the pan to heat through. Serve with lemon wedges, a green salad and crusty bread.