Melt the butter in a saucepan and add the onion. Saute the onion until lightly browned and add the flour. Cook until the flour is incorporated with the butter and onion, and is a light brown roux. Add the milk or cream and stir until thickened. Add the spinach and cook on medium low heat, stirring until the spinach is defrosted. Add the nutmeg and mix thoroughly in the spinach. Cook until hot.