Prep the beef: Pat the bulgogi beef dry with a kitchen towel to remove any blood. Set aside.
Make the marinade: In a bowl combine soy sauce, oligo syrup, sugar, minced garlic, mirin, sesame oil, a generous amount of black pepper, and grated or blended onion. Mix well. Taste it — it should taste strong and savory. If it seems bland add a little more soy sauce.
Marinate: Add the beef to the marinade and massage everything together until every piece is well coated. Cover and marinate in the fridge for at least 30 minutes — a few hours is better, overnight is best.
Prep the vegetables: Thinly slice the onion, carrot, shiitake mushrooms, and green onion. Set aside separately if you're doing it my mom's way, or mix into the beef if you're going the convenient route.
Cook: Heat a pan or grill pan over medium-high heat. If you're keeping the vegetables separate cook the beef first in batches — don't crowd the pan or it'll steam instead of sear. Once the beef is almost cooked through add the vegetables and cook together until everything is done. If you mixed the vegetables in with the marinade just cook everything together from the start.
Serve: Serve immediately over steamed white rice, with lettuce wraps and banchan on the side, or however you like it.