Heat 2 tablespoons of unsalted butter in a large saucepan or skillet over medium heat until melted and sizzling. A large saucepan will help keep onion slices from flipping out onto stove as you stir. This also gives the onion slices plenty of room to cook without being stacked on top of each other.
Cook the onions while gently stirring, until they are soft and starting to turn translucent, about 1–2 minutes.
Reduce the heat to medium-low and continue to cook onions, stirring every few minutes. Stirring gently prevents them from sticking to the bottom of pan and burning on edges.
Allow onions to cook until golden blonde in color, around 15–20 minutes. If onions are getting too brown for your liking, just reduce heat. Or continue to cook and stir an additional 10-15 minutes on low, for darker more caramelized onions.
Remove from heat and set aside for topping burgers later. You can place them in an air tight container and refrigerate for up to 1 week.