Mix garlic powder and 2 tablespoons each cornstarch and water; toss with beef. In a small bowl, mix soy sauce, brown sugar, ginger and the remaining cornstarch and water until smooth., In a large skillet, heat 1 tablespoon oil over medium-high heat; stir-fry beef until browned, 2-3 minutes. Remove from pan., In same pan, stir-fry broccoli and onion in remaining oil over medium-high heat until crisp-tender, 4-5 minutes. Stir soy sauce mixture; add to pan. Cook and stir until thickened, 1-2 minutes. Return beef to pan; heat through. Serve with rice.