In a large skillet, melt 1 tablespoon of unsalted butter and 1 tablespoon of olive oil over medium heat. Add the sliced yellow onions and stir every few minutes. After about 10 minutes, sprinkle with kosher salt to enhance flavor, and continue cooking for 30-40 minutes, stirring regularly, until they are golden brown and caramelized.

While the onions are caramelizing, soften 2 tablespoons of unsalted butter in a small bowl. Finely chop a few leaves of fresh sage and mix them into the butter, along with a pinch of salt.

Take two slices of sourdough bread and lay one slice down, non-buttered side facing up. Start layering the sandwich: add a slice of brie cheese, followed by a layer of cheddar, slices of honeycrisp apple, and the caramelized onions. Top it with another layer of cheddar and finish with the second slice of sourdough bread, buttered side facing up.

Heat a clean skillet over medium heat and add a bit more olive oil. Place the sandwich in the skillet, buttered side down, and cook for about 3-4 minutes until golden brown and crispy. Carefully flip the sandwich and cook the other side for another 3-4 minutes.

Once cooked, remove the sandwich from the skillet and let it cool for a minute before slicing it in half. Savor each bite and share with friends and family!