In a large saucepan, combine the chicken and water. Bring to boiling, reduce heat, and simmer for 40 minutes or until the chicken is tender.
Remove chicken from broth and let stand until cool enough to handle.
Skim fat from broth. Strain and reserve broth.
Remove chicken meat from bones. Cut into bite-size pieces.
In the same saucepan, cook celery and onion in hot oil for 4-5 minutes; add mushrooms and cover and cook for 5 to 10 minutes or until everything is tender, stirring now and then. Remove from heat.
Return broth to the saucepan.
Add the partially cooked wild rice to the chicken broth mixture. Stir in the bouillon granules and white pepper. Bring to boiling. Reduce heat and simmer, uncovered, for 15 minutes.
In a large separate saucepan, melt the butter. Stir in flour until it all clings together and is smooth. Add the milk all at once and stir and cook until it's bubbly and thick. Add some hot broth mixture to the white sauce mixture and stir until smooth; return all to the broth mixture.
Stir in the chicken pieces and the Sherry.
Makes 8 servings.