Mix balsamic vinegar, garlic, herbs, and 1 tablespoon olive oil together and pour over the mushrooms. Leave the mushrooms to marinate for about 20 minutes, turning them at least one to make sure that both sides have a chance to marinate in the mixture.
In the meantime, oil a frying pan and add the onion and peppers. Sauté until tender. Set aside.
Take out the marinated mushroom caps and cook about 6-8 minutes on each side until tender.
Toast the bread, add the mushroom caps and top with the pepper and onion mixture and tomatoes. Drizzle on balsamic vinegar if desired.
Kid Chef Tips: Younger children can add ingredients, stir the marinade, and flip the mushroom caps. Older children can help cut the veggies and toast the bread.