1/4 cup balsamic vinegar
1 tablespoon olive oil
1 garlic clove, minced
1 red bell pepper, cut in half and seeded
1 yellow bell pepper, cut in half and seeded
4 (4-inch) portobello mushroom caps
1/3 cup chopped fresh basil
1/4 teaspoon salt
1/4 teaspoon freshly ground black pepper
4 (2-ounce) Kaiser rolls
1/2 cup (4 ounces) soft goat cheese
Prepare grill to medium-high heat.
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.
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.
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.
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).