Begin by slicing your large yellow onions into ½ inch half moons. In a skillet over medium heat, melt 2 tablespoons of unsalted butter along with 1 tablespoon of olive oil. Once melted, add the onions, stirring well to coat. Allow them to cook for 40–50 minutes, stirring occasionally, until they turn golden brown and sweet, and remember to sprinkle in kosher salt after the first 10 minutes to enhance their flavor.

While the onions are caramelizing, mix 4 tablespoons of softened butter with 1 teaspoon of kosher salt and finely chopped fresh sage in a small bowl. Blend the ingredients until well-combined, then set aside to allow the flavors to meld together.

Once the onions are caramelized, assemble your sandwiches by taking a slice of sourdough bread and layering it with brie cheese, followed by a generous helping of cheddar cheese. Add crispy honeycrisp apple slices and the caramelized onions. Top with another slice of cheddar cheese and spread the sage butter on the other slice before topping it on your sandwich.

Heat 1 tablespoon of olive oil in the same skillet over medium heat. Place your assembled sandwiches buttered-side down onto the skillet. Grill for about 6 minutes, or until the bread is golden brown and crispy. Carefully flip the sandwiches, spreading any remaining sage butter on the top slice. Grill for an additional 6 minutes, checking frequently until both sides are crispy and the cheeses inside are beautifully melted.

After grilling, let them rest for a minute before cutting them in half to ensure a less messy bite. Serve warm with a side salad or hearty soup.