1. Cook bacon in a large skillet over medium heat until crisp. Drain on paper towels.
2. Spread 2 teaspoons mayonnaise on 1 side of each bread slice. Divide spinach evenly among 4 bread slices; top with 2 tomato slices and 2 bacon slices. Sprinkle sandwiches evenly with salt and pepper. Top with remaining bread slices, mayonnaise side down. Cut sandwiches diagonally into halves, if desired.