Slice your onion thin and heat olive oil in a large skillet over medium low heat. Add the sliced onions with a pinch of salt and cook for 30 to 40 minutes, stirring occasionally, until they're deep golden brown and sweet. Don't rush this step by turning up the heat or they'll burn instead of caramelize. If they start sticking to the pan, add a splash of water to deglaze. This is the most important step for Bacon and Caramelized Onion Grilled Cheese so take your time. Set the caramelized onions aside when done.

While the onions are caramelizing, cook your bacon in a separate skillet over medium heat until crispy, about 8 to 10 minutes depending on thickness. Flip occasionally so both sides cook evenly. Drain the cooked bacon on paper towels to remove excess grease. You can do this step ahead of time if meal prepping.

Butter one side of each bread slice generously, making sure to spread the butter all the way to the edges. On the unbuttered side of two slices, layer half the cheese, then the caramelized onions, crispy bacon broken into pieces, and the remaining cheese on top. The cheese on both the bottom and top helps everything stick together and ensures even melting. Top with the other bread slices, buttered side facing out. Press down gently to compact the sandwich slightly.

Heat a clean skillet (or the one you used for bacon, wiped clean) over medium heat. Place your assembled sandwiches in the pan and cook for 3 to 4 minutes per side, pressing down gently with a spatula for better contact with the pan. You want golden brown crispy bread and completely melted cheese throughout. If the bread is browning too fast before the cheese melts, reduce the heat to medium low and cook a bit longer. Don't rush this or you'll have burnt bread with cold cheese in the middle.

Remove the grilled cheese from the pan and let it rest for about 1 minute before cutting so the cheese sets slightly and doesn't all run out when you slice it. Cut diagonally with a sharp knife. Serve the Bacon and Caramelized Onion Grilled Cheese immediately while hot and crispy. The sweet onions, salty bacon, melty cheese, and crispy buttery bread make the perfect bite.