In a bowl, mix together the tuna, red pepper, scallion, mayo, yogurt, and vinegar. Season with salt and pepper to taste.
Spread the tuna over the two slices of bread and top each with 1-2 tablespoons of your favorite shredded cheese. Place on a baking sheet and bake for 10 minutes, or until the cheese is melted and just starting to brown around the edges, and the sandwiches are heated through. Allow to cool for a minute before cutting in half and serving.
Notes
In lieu of using half mayo, half yogurt, you can use all mayo or all yogurt. Similarly, the red pepper can be replaced with about 2 tablespoons of jarred pimentos.