1. Prepare grill to medium-high heat.
2. Combine balsamic vinegar, olive oil, and garlic in a large bowl. Add bell peppers and portobello mushrooms; toss gently to coat. Remove vegetables from vinegar mixture, and discard vinegar mixture.
3. Place bell peppers and mushrooms on a grill rack coated with cooking spray; grill 4 minutes on each side. Remove vegetables from grill; cool slightly. Cut bell peppers into thin strips. Combine bell peppers, basil, salt, and black pepper in a small bowl.
4. Cut rolls in half horizontally; spread cheese evenly over cut sides of rolls. Arrange 1 mushroom cap on bottom half of each roll; top each serving with about 1/3 cup bell pepper mixture and top half of roll.
5. Place sandwiches on grill rack coated with cooking spray. Place a cast-iron or other heavy skillet on top of sandwiches; press gently to flatten. Grill 3 minutes on each side or until bread is toasted (leave cast-iron skillet on sandwiches while they cook).
MyRecipes is working with Let's Move!, the Partnership for a Healthier America, and USDA's MyPlate to give anyone looking for healthier options access to a trove of recipes that will help them create healthy, tasty plates. For more information about creating a healthy plate, visit www.choosemyplate.gov.