Add wild rice to 2 cups water in a saucepan. Simmer for 45 minutes.
Saute' onion, celery, and mushrooms in butter in a large pan about 3 minutes or just until vegetables soften. Stir in flour, cooking and stirring until flour is mixed in but do not let it begin to brown. Slowly add hot chicken broth, stirring until all veg-flour mixture is well blended. Stir in drained, cooked rice and chicken. Season with salt, pepper, and thyme.
Heat thoroughly. Stir in half and half. Add sherry and heat gently but do not boil.