1. Cut meat into 6 rectangles. Place each piece between sheets of plastic wrap: pound to 1/8-inch thickness. Place in plastic food-storage bag.
2. Whisk olive oil, vinegar, oregano, garlic, red-pepper flakes, salt and pepper in small bowl. Add to meat. Refrigerate for 8 hours or overnight.Meanwhile, prepare filling:
3. Heat olive oil in medium-size skillet over medium-high heat. Add onion; reduce heat to medium. Cook 3 minutes, until softened. Stir in crumbs, egg, prosciutto, cheese, parsley, garlic, salt and pepper. Remove from heat.
4. Remove steaks from marinade; discard marinade. Divide filling among steaks; roll up jelly-roll style. Secure with wooden picks.
5. To cook, prepare outdoor grill with medium-hot coals, or heat gas grill to medium-hot (to oven-roast, see below).
6. Grill, covered, turning several times, 15 minutes or until beef is tender; be careful of flare-ups.
7. Heat marinara sauce in saucepan. Pour over roll-ups. Serve with spaghetti.