1Cut beef into beef strips. (Beef is easier to cut if partially frozen, about 1 1/2 hours.)
2Heat 12-inch skillet or wok over high heat. Add oil; rotate skillet to coat side.
3Add beef; stir-fry 2 to 3 minutes or until golden brown; drain. Stir in salsa, ketchup, brown sugar and mustard. Cook and stir 1 to 2 minutes or until hot. Serve in tortillas with remaining ingredients.