Generously butter one side of each slice of bread. On the unbuttered side, layer approximately 1/2 cup of the prepared tuna salad, followed by 2 to 3 slices of tomato, and top with 2 slices of cheddar cheese. Place another slice of bread on top, with the buttered side facing up. Repeat this process for the remaining sandwiches. Arrange the assembled sandwiches on a large baking sheet and bake in the preheated oven until the cheese is thoroughly melted and the bread is golden, about 5 to 8 minutes.