In a large skillet, heat olive oil over medium heat. Add the sliced onions with salt and pepper. Sauté for about 15 minutes until golden brown.

Remove the onions and in the same skillet, cook the bacon over medium heat for 5-7 minutes until crispy.

Assemble sandwiches with bread, cheese, bacon, caramelized onions, and another slice of cheese, then top with remaining bread.

Butter the outside of each sandwich evenly and preheat the skillet.

Grill the sandwiches for about 4-5 minutes on each side until golden brown.

Serve warm, letting them cool for a minute before slicing in half.