In a large mixing bowl, combine the drained canned tuna, finely chopped onion, and minced garlic. Stir in 2-3 tablespoons of mayonnaise, adjusting the amount based on your creaminess preference. Season with salt and ground black pepper to taste. Mix thoroughly until all ingredients are well-blended and the tuna mixture appears creamy, taking about 3 minutes.

Take two slices of your preferred bread—sourdough works beautifully—and spread half of the creamy tuna mixture generously over one slice. Layer two slices of fresh tomato on top, followed by two slices of your chosen cheese. Place a second slice of bread on top to complete the sandwich assembly, pressing lightly to help it hold together.

Using room temperature butter, spread a generous amount over both outer sides of the assembled sandwich. This will ensure a deliciously crispy, golden-brown crust when toasted.

Preheat a non-stick skillet over medium heat for about 2-3 minutes until hot. Carefully place the buttered sandwich in the skillet, cooking for 3-4 minutes per side. Press down gently with a spatula to encourage even toasting and check for a golden brown color before flipping.

Once both sides of the sandwich are golden brown and the cheese is gooey and melted, remove it from the skillet. Allow it to rest for a minute before slicing in half for easy handling. Serve your warm Tuna Melt Sandwich immediately, perhaps with a side of pickles or a fresh salad for a complete meal experience.