In a skillet over medium heat, add your bacon strips and cook for about 6-8 minutes until they are crispy and golden brown. Transfer the bacon to a plate lined with paper towels to drain excess fat.

In the same skillet with the bacon fat, add 2 tablespoons of butter and allow it to melt over low heat. Add the thinly sliced onions, stirring to coat. Cook for 15-20 minutes until onions are soft and caramelized.

Assemble the sandwiches by layering one slice of sourdough bread, a generous handful of cheese, crispy bacon, and caramelized onions. Top with another slice of cheese and bread.

Spread butter on the outside of both sandwich slices. Heat the skillet over medium heat and grill the sandwiches for about 3-4 minutes on each side until golden brown and cheese is melted.

Remove sandwiches from skillet and let them rest for about 1 minute. Cut in half and serve.