In a large skillet over medium heat, melt 2 tablespoons of unsalted butter. Add the sliced yellow onions, stirring them well to coat. Allow the onions to cook for 30 to 40 minutes, stirring occasionally, and sprinkle in a pinch of kosher salt after 10 minutes.

In a small bowl, mix 4 tablespoons of softened unsalted butter with finely chopped fresh sage and a pinch of kosher salt until well combined.

Place a slice of sourdough bread in a clean skillet over medium heat. Layer on slices of brie cheese, followed by sharp cheddar cheese, crisp honeycrisp apple slices, and caramelized onions. Top with another layer of cheddar cheese and finish with a second slice of bread.

Add a teaspoon of olive oil to the skillet and place the assembled sandwich buttered-side up. Cook for about 6 minutes until the bread is golden brown and crispy. Carefully flip and grill the other side until golden and the cheese is melted.

Remove the sandwich from the skillet, let it cool briefly, then slice in half and serve immediately, ideally with a warm bowl of tomato soup or a fresh salad on the side.