Slice two yellow onions into ½ inch half moons. In a large skillet, heat 2 tablespoons of unsalted butter and 1 tablespoon of olive oil over medium heat. Add onions, stir occasionally, add kosher salt after 10 minutes and cook for another 30-40 minutes until caramelized.

In a small bowl, soften 2 tablespoons of unsalted butter. Finely chop fresh sage and mix with butter and a pinch of kosher salt.

Assemble the sandwich with one slice of sourdough, brie cheese, cheddar, apple slices, caramelized onions, another slice of cheddar, and top with another slice of bread.

Preheat skillet and add olive oil. Spread sage butter on the outside of the sandwich. Grill for about 6 minutes per side until golden brown and the cheese melts.

Once both sides are golden, remove from skillet, let rest for a minute, slice in half, and serve warm.