Stir together the beef marinade ingredients in a medium bowl until cornstarch is dissolved. Add prepared beef into the marinade and coat well. Set aside.
Prepare the cooking sauce by mixing the sauce ingredients in a small bowl. Stir well.
Place ginger and garlic into the pan and quickly add beef, then immediately spread out beef in a single layer. Cook until beef reaches desired doneness (about 1-2 minutes).
Remove beef from the pan and set aside. Use the remaining oil in the pan, cook broccoli over high heat for 3 minutes, and then stir in mushroom. Cook for another 2 minutes or until the broccoli is tender-crisp.
Return beef to the pan and pour the cooking sauce over all the ingredients. Stir for another 30 seconds until even. Serve over rice.