Yet another allrecipes meal that I've taken and made my own with a few alterations.

INGREDIENTS
1 1/2 teaspoons butter, softened
2 - 3 slices whole wheat bread, or your favorite bread
2 tablespoons crumbled feta cheese
1 slice Cheddar cheese
1 tablespoon chopped red (or white) onion
1/4 tomato, thinly sliced

I personally don't really like tomato, so I left that out and added a little garlic. Almost everything I cook tends to have a little garlic in it at some point! You could also add Greek seasoning or even a little crushed red pepper. I love me a little spice...

DIRECTIONS
Heat a skillet over medium heat. Butter one side of each slice of bread. On the non buttered side of one slice, layer the feta cheese, Cheddar cheese, red onion and tomato. Top with the other slice of bread with the butter side out.
Fry the sandwich until golden brown on each side, about 2 minutes per side. The second side always cooks faster.

The first time that I tried this recipe, I made one sandwich and then tried to cook it while making a second and pretty much burnt it to a crisp! Just a warning!