Melt a tablespoon of the butter in a pot, add the onion and brown lightly over a low fire. Add half the remaining butter and mix well. Stir in the flour and continue to cook over a low fire, stirring, until the mixture foams. Whisk in the cider and bring to a boil. Season with salt, pepper and nutmeg and simmer for 7-8 minutes or until the sauce lightly coats a spoon. Stir in the cream and bring the sauce back almost to a boil. Remove the pan from the heat and whisk in the remaining butter, a piece at a time. Add a few drops of lemon juice and taste for seasoning. Don’t reheat the sauce or the butter will separate.