Cut beef into thin slices. Combine 1 tablespoon each of cornstarch, soy sauce, sugar, and garlic, then add the beef. Let stand for at least 15 minutes. Meat can also marinade over night. Set aside.
Combine 4 teaspoons cornstarch and 3 tablespoons soy sauce with 1 1/4 cup water. Set aside.
Heat 1 tablespoon of cooking oil in a large skillet over high heat. Add meat and cook for 1 minute. Remove from pan.
Heat remaining 2 tablespoons of oil in same pan. Add broccoli and onion and cook for 4 minutes till tender.
Stir in meat and soy sauce mixture and bring to boil. Cook and stir until sauce thickens.
Serve over rice.