Heat olive oil in a skillet over medium heat and add the onions with salt and pepper, cooking until caramelized and golden brown for about 15 minutes.

Cook the bacon in the same skillet until crispy, approximately 5-7 minutes, then drain on paper towels.

Assemble the sandwiches with a slice of cheese, crispy bacon, caramelized onions, and another slice of cheese between two slices of bread.

Butter the outer sides of the sandwiches and grill in a skillet over medium heat until golden brown and cheese melts, about 4-5 minutes per side.

Slice the sandwiches in half and serve warm.