Preheat the oven to 400°F (200°C). In a large bowl, whisk together mayonnaise, lemon juice, and red pepper flakes (if using).
Drain the tuna and add it to the mayonnaise mixture. Use a fork to break up the tuna into flakes. Add celery, pickles, red onion, and parsley, then 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 of tomato, and 2 slices of cheese. Top with another slice of bread, buttered side facing up. Repeat with the remaining ingredients and place on a large baking sheet.
Bake until the cheese is melted, for about 5 to 8 minutes.
Notes
Notes: For a crispy exterior, you can also toast the assembled sandwiches in a skillet over medium heat until golden brown on both sides.