1. Heat the olive oil in a skillet over medium heat. Add the sliced onions and season with salt and pepper. Cook until golden brown and caramelized, about 15 minutes.

2. In the same skillet, cook the bacon over medium heat until crispy and golden brown, about 5-7 minutes. Transfer to a paper towel-lined plate.

3. On one slice of bread, layer a slice of cheddar cheese, followed by bacon and caramelized onions. Top with another slice of cheddar cheese and the second slice of bread.

4. Spread 1 tablespoon of butter on the outside of each sandwich.

5. Preheat the skillet over medium heat. Place the sandwich in the skillet and cook until golden brown and cheese is melted, about 4-5 minutes per side.

6. Remove from skillet, let cool for a minute, then slice in half and serve warm.