Caramelize the onions:
In a skillet over medium heat, melt 2 tbsp butter and 1 tbsp olive oil. Add sliced onions and stir to coat. After 10 minutes, add 1 tsp kosher salt. Continue cooking for 30–40 minutes, stirring occasionally, until onions are golden and caramelized. Add a splash of water as needed to prevent drying. Set aside.

Make the sage butter:
In a small bowl, mix 4 tbsp softened butter with chopped fresh sage and a pinch of kosher salt. Set aside.

Prep the sandwich ingredients:

• Grate the cheddar cheese.

• Remove rind from brie and slice into ½-inch pieces.

• Thinly slice apple into ¼-inch slices.

Assemble each sandwich:

• Lay one slice of sourdough (unbuttered) on a surface.

• Add a layer of brie, ¼ of the cheddar, a few apple slices, ¼ cup caramelized onions, and another ¼ of the cheddar.

• Top with another slice of bread and spread sage butter on top.

• Repeat for second sandwich.

Grill the sandwiches:
Heat 1 tbsp olive oil in a skillet over medium heat. Place sandwiches buttered-side down. Spread remaining sage butter on the top side. Grill for ~6 minutes per side, or until golden and crispy. Serve warm.