In a large heavy skillet, heat the oil over medium heat. Add the onion and cook, stirring occasionally, until soft and golden, about 10 minutes. Stir in the vinegar, and add salt and pepper. Cook until the vinegar evaporates. Remove from the heat and cool. Preheat the broiler. On a baking sheet toast both sides of the bread. Divide the onion mixture and spread evenly over the top of the toasted bread. Cover the onions with overlapping slices of apples. Cover the apple slices with the cheese and broil the sandwiches until the cheese is melted and bubbly.
Yield: 4 servings