1. Preheat broiler.
2. Combine mayonnaise and pesto in a bowl, stirring well.
3. Arrange bread in a single layer on a baking sheet. Broil bread 2 minutes or until toasted. Turn bread over; place 1 cheese slice on each of 4 bread slices. Broil 1 minute or until cheese is bubbly. Spread about 2 teaspoons pesto mixture over each cheese-topped bread slice. Arrange 1/4 cup arugula and 2 tomato slices over pesto mixture; sprinkle tomato slices evenly with pepper and salt. Spread about 1 1/2 teaspoons of remaining pesto mixture evenly over one side of each remaining bread slice; place 1 slice, pesto-side down, on top of each sandwich.