Caramelize your onions first. Heat 2 tablespoon unsalted butter in a skillet on medium heat. Add the onions, stir every few minutes. At 10 minutes, add 1 teaspoon kosher salt. Continue to cook, stirring every few minutes for an additional 30-40 minutes until golden. Add 1 tablespoon water if the skillet starts to look dry.
Combine chopped sage with 4tbsp butter and 1 teaspoon kosher salt
To assemble, lay down a non-buttered piece of bread. Top with brie cheese, then cheddar cheese, then apples in a single layer slightly overlapping, then ¼ cup caramelized onions, then additional cheddar cheese, ending with the top piece of bread. Spread 1 tablespoon sage butter on the top piece of bread.
Heat a skillet on medium heat with 1 tablespoon olive oil. Place the sandwiches in buttered side down. Spread the remaining butter evenly on the tops of the exposed bread. Cook on each side for about 6 minutes until golden. Serve warm & enjoy!