Cook the bacon in a medium saucepan over medium heat until crispy, about 10-12 minutes.
Remove bacon and drain on paper towels.
Drain most of the bacon grease, leaving about 1-2 tablespoons in the pan.
Add onions and cook until caramelized, about 20 minutes.
Add balsamic vinegar, brown sugar, kosher salt, and black pepper.
Stir to combine and cook for another 10 minutes.
Return the bacon to the pan and stir.
Reduce the heat and simmer until the mixture is thick and jam-like, about 30 minutes.
Transfer the mixture to a food processor and pulse a few times until it's chunky but spreadable.