In a small bowl stir together soy sauce, sesame oil, and black pepper; set aside. In a 12-inch skillet or wok heat cooking oil over high heat for 1 minute. Add garlic; cook and stir for 1 minute. Add sweet pepper and onion; cook and stir for 3 minutes. Add mushrooms; cook and stir for 2 to 3 minutes more until vegetables are crisp-tender.
Add soy sauce mixture to skillet or wok. Cook and stir to coat vegetables. Transfer mixture to a serving dish. Sprinkle with sesame seeds. Makes 4 servings.