Start by making your tuna salad if you haven't already. In a mixing bowl, combine the 4 cups of tuna with mayonnaise, diced celery, chopped onion, a squeeze of lemon juice, and season with kosher salt and freshly ground black pepper to taste. Mix until well combined and set aside.
Take your slices of bread and lay them out on a clean surface. Spread a generous amount of tuna salad on four slices of bread. Top the tuna salad with a slice of cheese and a few slices of fresh tomato. Season the tomatoes with a pinch of salt and pepper for added flavor. Place the remaining slices of bread on top to create a sandwich.
Spread a thin layer of softened unsalted butter on the outside of each sandwich. This will help achieve that beautifully crispy crust when grilled.
Preheat your skillet or griddle over medium heat. Once hot, place the sandwiches in the skillet, buttered side down. Grill for about 3-4 minutes until the bread is golden brown and crispy. Carefully flip the sandwiches with your spatula and grill the other side for an additional 3-4 minutes, adding more butter if necessary.
Once both sides are golden brown, cover the skillet with a lid for the last minute of cooking. This helps the cheese melt perfectly without over-browning the bread.
Remove the sandwiches from the skillet and let them cool for a minute before slicing in half. Serve with your favorite side, whether it's a crisp salad or some crunchy chips. Enjoy your Best-Ever Tuna Melt while it’s still warm and gooey!
