In a large skillet, melt 2 tablespoons of unsalted butter over medium heat. Add thinly sliced yellow onions and stir occasionally for about 10 minutes. After they begin to soften, sprinkle with kosher salt. Continue cooking for an additional 30-40 minutes until the onions are golden brown and caramelized, stirring more frequently. If the onions appear dry, add a splash of water to keep them moist and cooking evenly.

While the onions caramelize, prepare the sage butter. In a small bowl, mix 4 tablespoons of softened unsalted butter with 1 teaspoon of kosher salt and a handful of chopped fresh sage. Blend until well combined, ensuring the herbs are evenly distributed in the butter.

Start assembling your delicious sandwich by laying one slice of sourdough bread on a clean surface. Spread a generous layer of brie cheese on the unbuttered side. Layer on slices of Honeycrisp apple, followed by a portion of the caramelized onions, and then a layer of sharp cheddar cheese. Finish with another slice of bread on top, keeping the buttered side facing out.

Heat 1 tablespoon of olive oil in the same skillet used for the onions over medium heat. Carefully place the assembled sandwich buttered-side down in the skillet, cooking for about 6 minutes, or until golden brown and crispy. Once the first side is browned, gently flip the sandwich. Optionally, spread some sage butter on top for extra flavor and cook for an additional 4-5 minutes until the cheese is melted and the bread is perfectly golden.

Once both sides are beautifully golden and the cheese is gooey, remove the sandwich from the skillet and let it rest for a minute. Slice the Caramelized Onion & Apple Grilled Cheese Sandwich in half. Serve warm, perhaps alongside a comforting bowl of tomato soup or a light salad.