Place the tuna in a medium mixing bowl and break it apart with a fork. Add the green onion, celery, mayo, salt, pepper, and Mozzarella to the tuna and mix well. Refrigerate the tuna mixture until needed.
Heat oven’s broiler. Line a baking sheet with aluminum foil.
Spread the butter generously over both sides of each piece of bread.
Place the bread on the prepared baking sheet and broil the bread until golden brown on top, about 2-3 minutes. Flip each slice of bread over and continue to broil until the second side is toasted, an additional 2-3 minutes. Remove the baking sheet from the oven.
Gently spread the tuna mixture evenly over the four slices of bread, making sure you get all the way to the edges. Sprinkle the Cheddar cheese evenly over the tops of the sandwiches.
Place the baking sheet with the sandwiches back into the broiler until the cheese is melted and bubbling, about 5-6 minutes.
Notes
Though I think this mixture as written is good … almost perfect … I also know not everyone has the same tastes! Experiment with …
Slices of tomato in the sandwich, or lettuce, carrots, avocado, pickles, etc
A dash of dijon mustard to add more flavor to the mixture, or hot sauce/sriracha
Relish or lemon juice in the mixture, again for more flavor
Herbs like parsley or oregano
There are lots of ways to customize this recipe! You can try …
White or red onion instead of green onion
Different cheeses: Colby jack, American, Asiago, Monterey jack, etc
Different kinds of bread: regular white, whole grain, etc
Avoiding the bread altogether and go for a lettuce wrap or tortilla