In a large skillet, melt 2 tablespoons of unsalted butter over medium heat. Add the sliced yellow onions and stir gently. Cook for about 10 minutes, then sprinkle with 1 teaspoon of kosher salt. Continue to cook for 30-40 minutes, stirring occasionally until the onions are deep golden brown and caramelized.

Combine the remaining 4 tablespoons of unsalted butter with 1 tablespoon of chopped fresh sage and 1 teaspoon of kosher salt in a small bowl. Mix until well blended.

Take one slice of non-buttered sourdough bread and layer with brie cheese, grated cheddar, honeycrisp apples, and caramelized onions. Top with more cheddar and another slice of bread.

Heat 1 tablespoon of olive oil in a skillet over medium heat. Place the sandwiches buttered-side down and grill for about 6 minutes or until golden brown.

Gently flip each sandwich using a spatula and cook the other side for another 6 minutes or until golden and the cheese is melted.

Remove the grilled cheese sandwiches from the skillet. Let them cool for a minute before slicing in half.