Preheat oven to 400 degrees. Line shallow baking sheet with foil coated with cooking spray.
Spread tomato and onion slices in a single layer. Drizzle with olive oil and sprinkle with salt & pepper and roast for 10 minutes.
Meanwhile butter one side of each bread slice and set aside.
Press roasted tomatoes with paper towels to absorb access moisture. Then layer onto bread slices. Top with mozzarella cheese slices, fresh basil, red onion, and a drizzle of balsamic vinegar.
Pan grill until golden brown on each side.