Easy Spinach Casserole

3 pkgs. frozen chopped spinach
Sm. can water chestnuts
1 pt sour cream
Cheddar cheese, grated, if desired
1 pkg. onion soup mix

Cook spinach, drain thoroughly. Add sour cream. Stir in onion soup mix and mix well. Slice water chestnuts and stir into mixture. Put in casserole. Sprinkle grated Cheddar cheese on top. Bake for 20 minutes in a 350 degree oven.