Place the diced onion and butter in a large saute pan or pot. Saute the onion for 2-3 minutes or until soft and translucent.Place the milk and cream in a separate smaller saucepan, heat to warm and set aside. Sprinkle 1 Tablespoon of flour over the cooked onions and butter. Stir and cook for 1 minute. Add the warm milk and cream to the mixture, whisk to prevent any lumps. Season with salt and pepper and add the nutmeg to the pan. Cook until you bring the mixture up in temperature to hot. Turn the burner off. Add the spinach to the pan, stir until the spinach is wilted but still has a gorgeous green color. Serve immediately.