In large saucepan, combine carrots, celery, Broccoli stems, green beans, Onion, salt and 1 cup grapefruit juice; bring to a boil. Cover; simmer 8 minutes. Add Broccoli flowerets. Cover. Cook 5 minutes or until vegetables are crisp-tender. Blend cornstarch with remaining ¾ cup grapefruit juice; pour over vegetables; bring to a boil, stirring constantly, until thickened. Serve at once with soy sauce, if desired.