Over high heat, preheat a sauté pan large enough so veal medallions do not overlap. Dredge medallions in flour, shaking off excess. Add olive oil to pan. and sear medallions on both sides until lightly golden.
Remove medallions and place on towel-lined plate. Drain excess oil from pan and add shallot. Sauté until translucent, about 1 minute. Deglaze pan with white wine, reducing by half. Scrape bottom with wooden spoon to release bits of veal that are stuck. Add veal stock, salt and pepper, rosemary and tomato purée, stirring to incorporate. Bring to a boil and reduce heat to medium-high. Return veal to pan.
Top each medallion with a slice of prosciutto and mozzarella cheese. Cover pan loosely and simmer for 2-3 minutes until cheese melts. Plate three medallions per serving and return sauce to heat. Add butter and whisk, mixing well. Simmer until melted and sauce has a creamy consistency. If it is too thick, add a little more stock. Spoon sauce over veal and serve hot.