Heart Healthy Balsamic Portobello Sandwich Recipe
Lightly coat a baking pan with nonstick cooking spray. Arrange mushroom caps and red onion slices on baking pan.
Drizzle balsamic vinegar and oil over tops of mushrooms. Season mushrooms with black pepper. Bake for 20 minutes.
Top mushroom caps with cheese and bake for 2 to 3 minutes more, or until cheese is melted.
With a spatula, transfer each mushroom cap to the bottom half of a roll. Top each mushroom with red onion, roasted pepper, spinach and tomato, if using.