My mother taught me this recipe, and it has become one of my favorite quick meals. I find that Granny Smith apples work best, as well as a crunchy seed bread.
Preheat a skillet over medium heat. Lightly brush one side of each slice of bread with the olive oil. Place 1 slice of bread, olive oil side down into the skillet, and arrange the apple slices evenly over the top. Sprinkle the Swiss cheese over the apple, then top with the remaining slice of bread, olive oil-side up. Cook until the bread is golden brown, then flip the sandwich over, and cook until the other side is golden brown and the cheese has melted, 1 to 2 more minutes.
Per Serving: 371 calories; protein 17.3g; carbohydrates 33.9g; fat 19g; cholesterol 33.1mg; sodium 338mg.