In a large bowl, combine cornstarch and water until smooth. Add the beef and mix until fully coated. Set aside for 2 minutes.
Heat 1 tablespoon oil in a large skillet on medium-high heat. Add beef and stir-fry for 3-5 minutes or until it's no longer pink. Remove from pan and set aside.
In the same pan, stir-fry garlic, broccoli, onion, bell peppers and mushrooms in the remaining oil, for 3-5 minutes. Return beef to pan.
In small bowl, whisk all the ingredients for the sauce and pour into pan. Stir-fry for 2-3 minutes or until everything is fully combines and the mixture as simmered.