Melt some butter directly on the pan over medium heat or add olive oil. You can spread butter or drizzle olive oil on the cut sides of the bun first if you want. Toast the bun.
Reheat the pan over high heat, and cook the marinated meat, loosening the meat and flipping them over once or twice. Transfer to another plate. If using cooked bulgogi, simply heat it up in the pan.
Reduce the heat to medium high, and add a couple of teaspoons of cooking oil to the pan. Stir fry the kimchi and optional onion slices until the kimchi turns slightly soft and deep in color.