Heat oven to 425°F. Heat olive oil and butter in cast-iron skillet over high heat. Add crushed garlic cloves to skillet. Use fork to spear garlic cloves to rub cloves over skillet surface while butter is melting. While skillet heats, pat steaks dry with paper towels. Season with salt and pepper to taste.
When skillet is very hot, remove and discard garlic cloves. Place steaks into skillet and sear about 4 minutes on each side. Remove from heat and let rest.
Quickly sauté tomatoes with onion and rosemary. Heat until onion is softened, then remove from heat and set aside.
Place 1 slice of prosciutto over each steak. Place steaks on cooking tray and place into oven. Cook to desired degree of doneness.
While steaks finish, place balsamic vinegar into small pot. Bring to boil and reduce to syrup.
When steaks are done, remove from oven and let rest 5 minutes. To serve, place steaks on platter. Top each steak with tomato/onion/rosemary mixture and drizzle with reduced balsamic vinegar. Slice into portions and serve with pasta, corn or polenta.