In a large skillet, melt 2 tablespoons of unsalted butter along with 1 tablespoon of olive oil over medium heat. Add the sliced yellow onions and stir to coat them evenly. Allow the onions to cook for 30–40 minutes, stirring occasionally, until they are soft, golden, and caramelized. At the 10-minute mark, sprinkle in 1 teaspoon of kosher salt. If the onions appear dry, add 1 tablespoon of water to help with the caramelization.

While the onions are caramelizing, take 4 tablespoons of room temperature butter and mix in 1 teaspoon of kosher salt and a handful of chopped fresh sage. Use a fork to thoroughly blend the ingredients and create a smooth sage butter.

Once the caramelized onions are done, spread a generous layer of sage butter on one side of a slice of sourdough bread. Layer the non-buttered side with slices of brie and cheddar cheese, followed by crisp apple slices, caramelized onions, and another layer of cheddar. Top with another slice of bread, buttered side up.

Heat 1 tablespoon of olive oil in the same skillet over medium heat. Carefully place the assembled sandwich in the skillet, buttered side down. Allow it to cook for approximately 6 minutes or until the bread is golden brown and crispy. Gently flip the sandwich and cook for an additional 6 minutes.

Once cooked to perfection, remove the sandwich from the skillet and allow it to rest for a minute. Cut the sandwich in half to reveal the melty goodness inside and serve warm.