1) Combine soy sauce, sherry, cornstarch, and sugar in medium bowl. Stir in meat and set aside for about 15 minutes.
2) Heat large skillet over medium heat. Add 1 tsp of the oil. Swirl to coat. Add half the meat in a single layer and leave until seared, about 1 minute.
3) Stir-fry meat until no longer pink, about 1 minute. Remove. Add 1 tsp oil and repeat with remaining meat.
4) Add remaining 2 tsp oil to pan. Add broccoli, bell pepper, and mushrooms. Stir-fry for 2 minutes.
5) Make a well in center and add white part of scallions, ginger, garlic, and pepper flakes. Stir-fry with vegetables for 1 minute.
6) Add ¼ cup water and cover pan until broccoli is just done, 3 minutes.
7) Return meat to pan and add stir-fry sauce. Cook until sauce thickens slightly, 1-2 minutes.
8) Sprinkle with green part of scallions and serve.