Stir together the marinade and marinate meat for 6 hours or overnight.

Drain marinade from meat and reserve.

Heat 2 tablespoons of the oil in a large nonstick wok skillet.

Stir-fry meat until cooked through, then drain it from the wok and place in a bowl nearby.

Add another tablespoon of oil to the wok (if needed) and stir-fry the onion and bell peppers until crisp-tender.

Whisk cornstarch into reserved marinade to use as a sauce.

Add the tomatoes and scallions to the pan and cook briefly, then stir in cooked beef to warm.

Add the sauce and cook until mixture thickens up (about 1 minute after it boils).

Serve the stir-fry with hot cooked rice.