Step 1: Caramelize the Onions
1. Heat the Pan: In a large skillet, heat the olive oil and butter over medium heat.
2. Add Onions: Once the butter is melted, add the thinly sliced onions. Season with a pinch of salt and pepper.
3. Cook Slowly: Cook the onions slowly, stirring occasionally, for about 15–20 minutes until they are soft, golden brown, and caramelized. If they start to stick, you can add a splash of water to deglaze the pan.

Step 2: Cook the Bacon
1. Cook Bacon: In a separate skillet, cook the bacon over medium heat until crispy, about 5–7 minutes. Flip halfway through for even cooking.
2. Drain: Once cooked, transfer the bacon to a paper towel-lined plate to drain excess grease.

Step 3: Assemble the Sandwich
1. Toast the Bread: If desired, lightly toast the bread slices in the same skillet used for the bacon to soak up some flavor.
2. Layer Ingredients: On one slice of bread, layer the crispy bacon, caramelized onions, and a slice of cheese. Add any optional toppings.
3. Top and Grill: Place another slice of bread on top. If you prefer, grill the sandwich in the skillet for a couple of minutes on each side until the cheese melts and the bread is golden brown.

Step 4: Serve
1. Slice and Enjoy: Cut the sandwich in half and serve warm.