Preheat your oven to 400°F (204°C). Gather your ingredients and prepare your baking sheet.

In a bowl, flake the drained tuna and mix in mayonnaise, lemon juice, celery, green onion, parsley, salt, and pepper until combined.

Spread butter on one side of each bread slice and lay them butter-side-down on a baking sheet.

Spoon tuna salad onto each slice of bread and add cheddar cheese on top, adding tomato if desired.

Place remaining bread slices butter-side-up on top of each sandwich. Bake for 5 to 8 minutes until cheese melts and bread is golden brown.

Remove from oven, let sit briefly, slice if desired, and serve warm.