Slice the large yellow onions into ½ inch half moons. In a skillet, combine 2 tablespoons of unsalted butter with 1 tablespoon of olive oil, heating over medium heat. Add the sliced onions, stirring to coat. Allow the onions to cook for 40-50 minutes, stirring occasionally until they turn a rich golden brown. Add 1 teaspoon of kosher salt after 10 minutes to enhance sweetness; add a splash of water if they begin to dry out.

While the onions are caramelizing, blend together 4 tablespoons of softened unsalted butter with 1 teaspoon of kosher salt and a handful of chopped fresh sage. This fragrant sage butter can sit at room temperature to let the flavors meld.

On a clean surface, lay one slice of sourdough bread down. Start layering by adding a slice of brie, followed by sharp cheddar, and then thinly sliced honeycrisp apples. Top this combination with the caramelized onions. Place another slice of cheddar on top before adding a second slice of sourdough.

Heat a skillet over medium heat and pour in 1 tablespoon of olive oil. Carefully place the assembled sandwich, buttered side down, into the skillet. Grill for about 6 minutes, until the bottom is golden brown and the cheese begins to melt. Gently flip the sandwich, spreading the remaining sage butter on top, and grill for another 6 minutes.

Once grilled to perfection, carefully remove the sandwiches from the skillet and let them cool for a minute. Slice each sandwich in half and serve warm.