Clean and trim spinach. In a large, heavy saucepan, melt 1 tablespoon butter. Add spinach and cook over brisk flame for 5 minutes, stirring constantly. Remove pan from fire.
Add pepper, salt, nutmeg, and sugar. Sprinkle with flour. Mix well. Return pan to fire and cook for 2 minutes, stirring constantly. Again remove pan from fire. Stir in cream.
Return pan to fire and bring to boil, stirring constantly. Lower flame. Cover and simmer for 20 minutes.
Off fire, add another 1 tablespoon butter cut into small pieces, stirring until butter is melted and well blended.
In bottom of ovenproof dish, spread 3 tablespoons Mornay Sauce. Add spinach. Cover with remaining Sauce. Sprinkle with grated cheese. Add 2 remaining tablespoons melted butter. Bake at 500ºF degrees for 3 minutes, or until the top is golden.