Combine first 3 ingredients. Spread mayonnaise mixture on 1 side of each bread slice. Top each of 4 slices with 2 tomato slices, and sprinkle with pepper. Top with remaining bread slices. Coat sandwiches with cooking spray.
Place a large nonstick skillet over medium-high heat until hot. Add sandwiches, 2 at a time, and cook 3 minutes on each side or until lightly browned. Serve immediately.