In a large bowl, whisk mayonnaise, lemon juice, and red pepper flakes (if using).
Drain tuna, then add to mayonnaise mixture. Using a fork, break up tuna into flakes. Add celery, pickles, onion, and parsley and toss to combine; season with salt and pepper.
Spread butter on one side of each bread slice. Top an unbuttered side with about ½ cup tuna salad, 1 to 2 slices tomato, and 2 slices of cheese. Top with another slice of bread buttered side up. Repeat with remaining sandwiches.
Preheat a large nonstick skillet over medium heat. Add 2 sandwiches, arranging cheese side closest to pan, and cook until golden brown, 2 to 3 minutes. Flip and continue to cook until golden brown on the other side and cheese is melty, 2 to 3 minutes more. Transfer sandwiches to a plate.
Repeat with remaining sandwiches. Serve warm.
Notes
Feel free to customize the fillings and adjust seasoning according to your taste.