Heat a tablespoon of olive oil in a skillet over medium heat. Add thinly sliced onions, seasoning them lightly with salt and pepper. Sauté the onions for about 15 minutes, stirring occasionally, until they turn golden brown and sweetly fragrant. Remove the caramelized onions from the skillet and set aside for later.

In the same skillet, add the bacon strips and cook over medium heat for about 5-7 minutes, flipping occasionally until they become crispy and golden. Once cooked, transfer the bacon to a plate lined with paper towels to drain excess grease.

To create your delicious grilled cheese sandwiches, start with two slices of your chosen bread. Layer one slice with a generous portion of cheese, followed by crispy bacon, caramelized onions, and another slice of cheese on top. Place the second slice of bread over the top.

Spread a thin layer of unsalted butter on the outer sides of each sandwich. This helps create a beautifully golden-brown crust.

Preheat the skillet over medium heat and place the assembled sandwiches in the pan. Cook for about 4-5 minutes on each side, gently pressing down to ensure even browning.

Once nicely golden and crispy, remove the sandwiches from the skillet and let them cool for a minute. Slice each sandwich diagonally and serve warm.