Freeze the beef for about 30 minutes to make it easier to slice thinly. This step is optional but highly recommended for achieving tender beef slices.
In a mixing bowl, whisk together the soy sauce, brown sugar, sesame oil, minced garlic, and grated ginger until the sugar has dissolved.
Take the beef out of the freezer and slice it against the grain into thin strips. Cutting against the grain helps to ensure the beef remains tender.
Place the sliced beef in a ziplock bag or a shallow dish and pour the marinade over it. Mix well, making sure all the beef is coated.
Marinate in the refrigerator for at least 1 hour, or overnight for maximum flavor.
Heat the vegetable oil in a large skillet or grill pan over medium-high heat.
Add the marinated beef and cook for about 3-4 minutes per side, or until the beef is cooked through and slightly caramelized.
Optional: If using sliced mushrooms or onions, you can add them to the pan along with the beef and cook until tender.
Sprinkle the cooked beef bulgogi with green onions and toasted sesame seeds for added freshness and crunch.
Serve hot with steamed rice and enjoy!