Heat olive oil in a skillet over medium heat. Add onions seasoned with salt and pepper. Cook for about 15 minutes until golden brown.

In the same skillet, add bacon slices in a single layer. Cook for 5-7 minutes until crispy. Transfer to a paper towel-lined plate.

Layer one slice of bread with cheese, followed by bacon and caramelized onions. Add another slice of cheese and top with the second slice of bread.

Spread butter on the outside of each sandwich. This will help achieve a golden crust.

Grill the sandwiches in the skillet over medium heat for 4-5 minutes on each side until golden and crispy.

Remove from skillet, let cool slightly, and slice in half diagonally. Serve immediately.