Slice the large yellow onions into ½ inch half moons. In a skillet over medium heat, melt 2 tablespoons of unsalted butter with 1 tablespoon of olive oil. Add the sliced onions, stirring to coat. Cook for 40–50 minutes until golden brown, adding 1 teaspoon of kosher salt at the 10-minute mark.

Blend 4 tablespoons of softened butter with 1 teaspoon of kosher salt and a handful of finely chopped fresh sage to create sage butter. Set aside.

On a clean surface, place a non-buttered slice of sourdough bread. Layer with brie cheese, cheddar, apple slices, and caramelized onions. Top with another layer of cheddar and spread the second slice of bread with sage butter.

In a heated skillet over medium heat, add 1 tablespoon of olive oil and place the sandwiches buttered side down. Grill for 6 minutes until golden brown, then flip and spread sage butter on the top, grilling for another 6 minutes.

Remove sandwiches from the skillet, let cool briefly, then cut in half. Serve warm, optionally with a side salad or hearty soup.