In a large pot, place chicken, onion and enough water to cover. Bring to a boil, then reduce heat and simmer. Stir chicken occasionally and cook until no pink is visible (add water if it starts to boil dry). Add carrots, celery, bay leaves and water. Stir in stock. Cook on medium heat, partially covered, for about 30 minutes - until carrots are tender. Add mixed vegetables and egg noodles. Simmer until noodles are tender. Remove bay leaves right before serving. (Note: Noodles can be replaced by 1/2 cup rice; cook until rice is tender.)