Asian Friendship Soup

1 Large onion, thinly sliced
2 Tablespoons vegetable oil
1 1/2 Pounds beef top sirloin steak, cut into thin slices
1 Cup celery, sliced
2 (14 1/2 oz) Cans beef broth
1 Tablespoon cornstarch
2 Tablespoons soy sauce
1 Pound frozen mixed vegetables
2 Cups fine egg noodles, cooked
10 Ounces fresh spinach, torn
1/4 Teaspoon pepper
Chow mien noodles, optional

In a Dutch oven or soup kettle, saut� onion in oil until tender. Remove and set aside. In the same pan, stir-fry beef, a few slices at a time, until no longer pink. Add celery; stir-fry for 2 to 4 minutes or until crisp-tender. Return onion and meat to pan. Add broth.
In a small bowl, combine cornstarch and soy sauce until smooth. Gradually add to pan. Bring to a boil; cook and stir for 2 minutes or until thickened. Stir in vegetables, egg noodles, spinach and pepper; heat through. Garnish with chow mien noodles if desired.