In a large skillet, melt 2 tablespoons of butter over medium heat. Add in the sliced yellow onions, stirring occasionally. Sprinkle 1 teaspoon of kosher salt after about 10 minutes. Cook for 30-40 minutes until caramelized, adding water if necessary.

Mix 4 tablespoons of softened unsalted butter, 1 tablespoon of chopped fresh sage, and 1 teaspoon of kosher salt in a small bowl to prepare sage butter.

Layer 2 ounces of brie cheese, 4 ounces of grated cheddar cheese, slices of honeycrisp apple, and spoon caramelized onions between 2 slices of sourdough bread, with sage butter on the outer sides.

Heat 1 tablespoon of olive oil in a skillet over medium heat. Place the sandwich buttered side down in the skillet and cook for about 6 minutes until golden and crispy, then flip and cook the other side for 5-6 minutes.

Once both sides are golden brown and cheese is melted, slice in half and serve warm.