Just like the corner store used to make.
Turn the oven up to 400 F. In a big bowl, whisk together the mayo, lemon juice, and red pepper flakes if you're using them.
Drain the tuna, then it add to mayo mix, using a fork to break the tuna up into flaky bits. Add in the celery, pickles, red onion, and parsley and toss it all around to combine. Season the mix with salt and pepper to your preference.
Butter one side of each slice of bread, topping the unbuttered sides with 1/2 cup of the tuna mix, 2 to 3 slices tomato, and 2 slices of cheese. Top that with the other slice of bread, butter side up. Keep making sandwiches until the ingredients are used up, then bake all of them in the preheated oven until the cheese melts, 5 to 8 minutes.