Thaw spinach and squeeze out as much liquid as possible. Coarsely chop spinach. Peel and chop onion. In a 10- to 12-inch frying pan, frequently stir onion, 1/4 cup water, and butter until onion is slightly browned, about 5 minutes. Remove from heat and mix flour with onion, then smoothly stir in chicken broth and whipping cream. Season with nutmeg. Stir over high heat until boiling, about 2 minutes. Add spinach, mix well, and season to taste with salt and pepper. Use hot or cold.