Heat the butter in a 12-inch saute pan over moderate heat. Add the onions and cook until soft, about 8 minutes. Whisk in the cream cheese and let melt. Fold in the spinach until evenly combined. Add up to 3 tablespoons water if the mixture seems too dry. Sprinkle the nutmeg on top and season with salt and pepper.
BYOC: Lemon zest and juice would bring a little bit of sunshine to this dish. Just add with the salt and pepper.