1. Heat a large skillet over medium heat. Add 2 tablespoons oil; swirl to coat. Add prosciutto; cook 4 minutes or until crisp, stirring occasionally. Remove prosciutto with a slotted spoon. Drain on paper towels. Add bread to pan; cook 3 minutes or until browned, stirring frequently. Combine prosciutto and bread. Add remaining 1 tablespoon oil to pan. Add tomatoes; cook 5 minutes or until skins begins to split, stirring frequently. Pour tomatoes and olive oil into a small bowl. Sprinkle with salt and pepper.
2. Preheat grill to medium-high heat.
3. Coat cut sides of lettuce with cooking spray. Place lettuce, cut side down, on a grill rack coated with cooking spray. Cook 2 minutes or until well marked. Place 1 lettuce half on each of 4 plates. Divide prosciutto mixture and tomato mixture among servings. Top each serving with 1 tablespoon onions and 1/2 ounce cheese.