In a 12-inch skillet, cook sausage and onion until sausage is brown. Drain off fat. Add sliced mushrooms and water chestnuts and cook until mushrooms are tender. Stir in flour and pepper. Add chicken broth and milk all at once. Cook and stir until mixture is thickened and bubbly. Cook and stir for 1 minute more. Remove skillet from heat.
Meanwhile, in a saucepan, cook long grain and wild rice mix according to package directions.
Toss together the sausage mixture, rice, and chicken or turkey. Transfer to a 2-1/2- or 3-quart casserole.
Bake, uncovered, in a 350 degree F oven for 25 to 30 minutes or until heated through. Makes 6 servings.