For sauce, in a small bowl stir together salsa and cornstarch. Set aside.
Stack tortillas; wrap in foil. Heat in a 350 degree F oven about 10 minutes or until warm. Or, place tortillas, half at a time, between layers of microwave-safe paper towels; micro-cook on 100 percent power (high) for 1-1/2 to 2 minutes or until warm.
Pour cooking oil into a wok or large skillet. (Add more oil as necessary during cooking.) Preheat over medium-high heat. Stir-fry sweet pepper, corn, and green onions for 2 minutes or until sweet pepper is crisp-tender. Remove vegetables from the wok.
Crumble beef into the hot wok. Stir-fry for 2 to 3 minutes or until brown, stirring only as necessary. Drain off fat. Push meat from the center of the wok.
Stir sauce. Add sauce to center of the wok. Cook and stir until thickened and bubbly. Return cooked vegetables to the wok. Stir all ingredients together to coat with sauce. Stir in tomatoes; reduce heat. Cover and cook for 1 minute more or until heated through. Serve immediately with tortillas. Sprinkle with cheese. Garnish with cilantro, if desired. Makes 4 servings.