Place a large nonstick skillet coated with cooking spray over medium-high heat; then add the chicken. Cook it for 5 minutes on each side. After that reduce heat to low. Cover the pan and cook for 5 minutes or until done. Check if it is cooked by using the thermometer. Then remove it from heat.
Once that has cooled a little bit, cut the chicken diagonally into thin slices.
Spread a tablespoon of mayonnaise on each bread slice. Then top with lettuce leaf, chicken breast, and tomato slices, and put the remaining bread slices on top. After that, serve them warm.
