In a large bowl, combine the brown sugar, soy sauce, lime juice, 8 cloves of garlic, salt and black pepper to taste. Do not go crazy with the salt, as the soy sauce is already salty! Add the beef and let it marinate for 1 hour.

Preheat a large wok over high heat. Add the vegetable oil and, when simmering, add the remaining 2 cloves of garlic. Once fragrant, add the beef cubes, working in 2 to 3 batches if needed. Sear on all sides, shaking the pan, until medium rare, 1 to 2 minutes. Remove the beef to a plate and reserve.

Toss the onions in what's left of the marinade in the bowl. Add the onions to the wok and sauté until soft and beginning to brown. Add the beef (and its juices) back to the pan and toss with the onions.

Remove from heat, sprinkle with sliced scallions and serve over rice or salad.