1. Preheat oven to 400° F.
2. Toast your bread in your toaster, or in the oven.
3. In a large bowl, combine together mayonnaise, lemon zest and juice, and red pepper flakes (if using).
4. Drain tuna then add to mayonnaise mixture. Use a fork to break up tuna into flakes. Add onion, and parsley and toss to combine. Season with salt and pepper.
5. Top a side of bread with approximately 1/2 cup of tuna salad, and 2 slices of cheese. Repeat with remaining ingredients and place on a large baking sheet.
6. Bake until cheese is melty, 5 to 8 minutes.
7. Serve immediately and enjoy it warm.