Heat the oil in a large nonstick skillet coated with cooking spray over medium heat. Add onion and bell pepper; sauté 4 minutes. Add mushroom caps, and sauté 5 minutes or until tender. Remove pan from heat. Stir in the vinegar, capers, salt, and black pepper.
Spread about 2 tablespoons cheese over each bread slice, and top each with 3 tomato slices and 2/3 cup mushroom mixture.