Heat a large skillet over medium heat. Reduce the heat to medium-low and add the onion, salt and pepper. Stir to coat. Slowly caramelize the onion, stirring occasionally, for about 40 minutes, or until golden brown. (See why caramelizing onions for real takes patience!)

Once the onions are caramelized, transfer them to a small bowl. Return the skillet to medium heat; melt 2 teaspoons of butter in the skillet and add the apple slices.

Cook the apple slices until they're tender, about 5 minutes. Stir in the maple syrup and cook for 3-5 minutes more, or until the liquid is absorbed and the apples are just beginning to brown on the edges. Remove from heat.

Heat an indoor grill or panini press to medium heat. While the grill is heating up, place 4 pieces of bread on a cutting board. Top the slices with half the cheese, then the caramelized onions, maple apples, bacon, remaining cheese, and remaining bread.

Spread the tops of the sandwiches with 1/2 teaspoon of butter. Place the buttered side down onto the panini press or grill, then butter the opposite sides with the remaining butter.

Gently press the top of the grill down and grill the sandwiches for about 5 minutes, or until they have nice grill marks and the cheese is melted. Serve immediately.