Heat 2 tablespoons of olive oil in a skillet over medium heat. Add thinly sliced onions, seasoning with salt and pepper. Cook for about 15 minutes until golden brown.

In the same skillet, add bacon strips and cook over medium heat for 5-7 minutes until crispy. Remove and let drain on paper towels.

Layer a generous amount of grated cheese on one slice of bread, followed by bacon and caramelized onions. Add another layer of cheese and top with the second slice of bread.

Spread unsalted butter on the outer sides of each sandwich. Cook in the preheated skillet over medium-low heat for 4-5 minutes per side until golden brown.

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