Bring broth to a boil in a large stockpot over high heat. Meanwhile, mix the cornstarch with the water in a small bowl; set aside. Add the cabbage, mushrooms, green onions, soy sauce, and gingerroot to the boiling broth. When the broth mixture returns to a boil, stir in the cornstarch mixture. Let the mixture boil for about 3 minutes. Add the tofu strips, vinegar, and pepper. Taste and add more pepper or vinegar if desired. Reduce heat to simmer. Slowly drizzle the egg substitute into the simmering soup, stirring gently. Remove from the heat, stir in the sesame oil, and serve.