Slice the onions into 1/3-inch strips. Chop the garlic cloves. Slice the mushrooms, if not pre-sliced.
Preheat a large pan over medium heat and add shaved beef steak. Quickly separate the shaved beef pieces. To the beef, add sliced onions, chopped garlic, soy sauce, and sliced baby bella mushrooms. Then add sesame oil, black pepper, and brown sugar.
Cook, covered, for 15 minutes, or until all the ingredients are cooked through.
Chop scallions and add them to the bulgogi. Serve warm over a bowl of rice or with rice on the side.