01 -
In a medium bowl, combine the drained tuna, chopped celery, roasted red pepper, minced scallion, Greek yogurt, Dijon mustard, and ground pepper. Stir until all ingredients are well mixed.
02 -
Spread 1 teaspoon of mayonnaise (or softened butter) on one side of each slice of bread. Flip 2 of the slices over so the mayonnaise side is down, and top each with half of the tuna mixture. Place 1 slice of cheddar cheese on top of each portion of tuna, then top with the remaining bread slices, mayonnaise-side up.
03 -
Heat a large skillet over medium heat. Place both sandwiches in the pan and cook until golden brown on the bottom, about 3-5 minutes. Flip the sandwiches and cook until the second side is golden and the cheese is melted, about 3-5 minutes more.
04 -
Remove from the skillet and serve immediately while hot and melty.