Cook spaghettini according to package directions. Drain well and keep warm.
Meanwhile, for sauce, in a small bowl stir together water, soy sauce, cornstarch, black pepper, and red pepper. Set aside.
Spray a cold wok or large skillet with nonstick spray. Heat over medium-high heat. Add pea pods, green or red sweet pepper and garlic, stir-fry 1 minute. Add mushrooms; stir-fry 1 to 2 minutes more or until vegetables are tender. Remove vegetables from wok.
Add oil to wok. Stir-fry beef in hot oil for 3 to 4 minutes or until done. Push beef to sides of wok. Stir sauce and pour into the center of wok. Cook and stir until thickened and bubbly. Return vegetables to wok; cook and stir all ingredients 1 minute. Toss with spaghettini. Makes 4 servings.