This is a delicious sandwich that will satisfy your bacon and cheese craving, with caramelized onion for extra flavor. This recipe is a little time consuming and you have to keep your eye on the onion and bacon, but it's well worth it.
Place bacon on a baking sheet lined with foil. Place in cold oven and set oven to 400°F
While the bacon is cooking, heat a skillet or small saucepan to medium heat. Add butter. Once the butter starts to melt, add the onion. Cook onion, stirring frequently, about 15-20 minutes or until a caramel color develops and onions are very soft. Add more butter as needed.
After 15 minutes, check on the bacon. If it is cooked to your liking, remove the baking sheet from the oven and place bacon on a plate lined with paper towels. Set aside.
1-2 minutes before the onions have finished cooking, place cheese slices on top of the onions and allow to melt.
If using, spread mayonnaise on each slice of bread. Place bacon on one slice of bread, then top with the onion and cheese. Top with other slice of bread to form a sandwich.