Pre-heat oven to 400°.
In a large bowl, whisk together mayonnaise, lemon juice and red pepper flakes.
Drain tuna; add to mayonnaise mixture.
Use a fork to break up tuna into flakes.
Add celery, pickles, red onion and parsley; toss to combine.
Season with salt and pepper.
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 cheese slices.
Top with another slice of bread, buttered side facing up.
Repeat with remaining ingredients and place on a large baking sheet.
Place in pre-heated oven and bake until cheese is melty; 5 to 8 minutes.
