I just cooked them in the same pan along with the steak. But first I sealed the meat at a high temperature then I turned off the heat completely. My pan is cast iron so it keeps the heat. After turning it off, I threw the onions into the pan and just added 3 tbsp of butter. Cooked it for around 7-8 minutes until they look caramelized and soft. Since it is also blended with the juice of the meat it looks extra caramelized.