Transfer beef and onion to a large mixing bowl and mix to combine. Set aside.
In a small mixing bowl, combine soy sauce, brown sugar garlic and ginger. Stir to dissolve sugar.
Bring a heavy bottomed sauté pan to medium high heat. Add sesame oil to pan to heat. Add beef and onion mixture and sauté until meat has begun to brown. Work in batches if needed to discourage “steaming” of your beef.
After browning, pour liquid mixture into the pan. If working in batches, add the other browned beef and gently fold to incorporate sauce into the mixture. Reduce to medium-low heat and allow the sauce to reduce, about 3 minutes.
Remove from heat after reduction has completed. Top with sesame seeds and scallions. Immediately serve by dividing beef mixture into tortillas or lettuce wraps. top with cilantro, cabbage and extra scallions and a squeeze of lime juice. Enjoy with pickled vegetables and rice.