Heat 12-inch nonstick skillet over medium-high heat. Spray beef with cooking spray; add to skillet. Cook 3 minutes, stirring occasionally, until browned.
Meanwhile, in small bowl, mix hoisin sauce, water, gingerroot, garlic, oil and pepper flakes. Add sauce mixture and onions to steak; cook 1 to 2 minutes or until beef is desired doneness and sauce is slightly reduced (do not overcook). Serve immediately.