Apple and Brie Grilled Cheese with Caramelized Onions

Ingredients:
2 slices sourdough bread
1/4 cup butter, softened
2 slices Swiss cheese
2 slices brie cheese
1/2 large onion, thinly sliced
1 tablespoon brown sugar
1 teaspoon balsamic vinegar
Instructions:

Caramelize the onions:
In a skillet over medium heat, add the butter and onions. Cook until the onions are soft and golden brown, about 10-15 minutes. Stir in the brown sugar and balsamic vinegar, and cook for another minute.

Assemble the grilled cheese:
Butter one side of each bread slice. Place one slice of Swiss cheese on one slice of bread, followed by the brie cheese and caramelized onions. Top with the remaining Swiss cheese and bread slice.

Cook:
Heat a skillet over medium heat. Place the sandwich in the skillet and cook until golden brown on both sides and the cheese is melted, about 3-4 minutes per side.

Serve:
Cut the sandwich in half and serve immediately.

For a twist: Add a dollop of apple butter or a sprinkle of cinnamon sugar to the sandwich before serving.