Slice the large yellow onions into ½ inch half moons. Melt 1 tablespoon of unsalted butter and 1 tablespoon of olive oil over medium heat in a skillet. Add the onions and stir to coat them evenly. Cook for 40–50 minutes, stirring occasionally, until golden brown. Sprinkle kosher salt at the 10-minute mark.

In a small bowl, blend 2 tablespoons of softened unsalted butter with 1 teaspoon of kosher salt and 1 tablespoon of finely chopped fresh sage using a fork until smooth. Set aside.

Assemble the sandwich on a clean cutting board. Lay one slice of sourdough bread, layer with brie cheese, followed by cheddar cheese, honeycrisp apple slices, caramelized onions, more cheddar cheese, and top with another slice of sourdough. Spread sage butter on the outer side of the top slice.

Heat a skillet over medium heat with a drizzle of olive oil. Place the sandwich in the skillet and grill for about 6 minutes until golden-brown. Flip the sandwich, add sage butter to the top slice, and grill for an additional 6 minutes until both sides are golden brown and the cheeses melt.

Remove from skillet and cool for a minute. Cut the sandwich in half diagonally and serve warm.