In a large bowl, whisk together mayonnaise, lemon juice, and red pepper flakes (if using). Use a fork to break up tuna into flakes. Butter one side of each bread slice. Spread untoasted side of each slice of bread with about 1/3 cup of the tuna salad, 1 slice of the cheese, 2 slices tomato and 1 of the remaining slice cheese. Tuna melts are sandwiches that are made with canned tuna that is mixed into a tuna salad. Drain tuna then add to mayonnaise mixture. You then take that tuna salad filling and either pile it onto a bun, or sandwich bread and melt it … Directions Preheat oven to 400°. Return sandwiches to oven; broil 3 to 5 minutes or until cheese is melted.