Heat stock with frozen peas and carrots over high heat until hot, reduce heat so that broth simmers gently. Blend soy sauce into cornstarch in cup until smooth, stir into broth. Cook and stir 2 minutes or until soup boils and thickens slightly. Beat egg with egg white in small bowl, stirring constantly in one direction. Slowly pour egg mixture in a thin stream into soup. Add mushrooms, salt, and pepper to taste. Serve by itself or with chow mein noodles.