Preheat oven to 400°. In a large bowl, whisk together mayonnaise, lemon juice, and red pepper flakes (if using).
STEP - 2
Drain tuna then add to mayonnaise mixture. Use a fork to break up tuna into flakes. Add celery, pickles, red onion, and parsley and toss to combine. Season with salt and pepper.
STEP - 3
Butter one side of each bread slice. Top an unbuttered side with approximately 1/2 cup of tuna salad, 2 to 3 slices tomato, and 2 slices of cheese. Top with another slice of bread, buttered side facing up. Repeat with remaining ingredients and place on a large baking sheet. Bake until cheese is melty, 5 to 8 minutes.