Combine the marinade ingredients in a small bowl. Add the beef, stir to coat, and set aside for 30 minutes. Combine the sauce ingredients in a small bowl; set aside.
Place a wok over high heat until hot. Add 2 TBS of the canola oil, swirling to coat sides. Add the beef and stir fry until barely pink, about 2 minutes. Remove the beef, reserve. Add remaining 1 TBS oil to the wok. Add the garlic and onion and cook until the onion is soft, about 1 minute. Add the bell pepper; stir for 30 seconds. Stir in the tomatoes and the sauce mixture and stir well. Return the beef to the wok and add the cornstarch solution. Cook, stirring, until the sauce boils and thickens.
