he butter in a pot over medium heat. (Why yes, that is a lot of butter, but it’s used to fully submerge the onion while it cooks; you will not eat 2 cups of butter in this burger.) Add the onion, turn the heat to low, and gently cook at a bare simmer until the onion is tender, about 20 minutes. The onion should be cooked but still al dente, so there’s some texture and a slight hit of sharpness yet not enough that you’ll taste onion the rest of the day. Remove the onion from the butter and drain on a 