Melt butter. Add spinach and cook until all the liquid has evaporated. Stir in the cream of mushroom soup. Remove from heat and set aside. Combine white sauce, cheese, and lemon juice in saucepan. Cook over medium heat, stirring until cheese melts. Remove cartilage from crab meat. Add to sauce along with water chestnuts.
In 10" casserole, layer the spinach and mushroom soup using 1/2 of the mixture. Top with half of the crab mixture. Repeat layer. Top with Parmesan. Bake at 300 degrees for 1 hour.