In a pan of boiling water, cook the spinach for just a few minutes until vibrant in colour, wilted and tender. Drain well and press between two plates to remove any excess water. Rub through a sieve or muslin to form a purée.
In a saucepan, melt the butter and add the spinach purée. Cook for a few minutes then add the cream and heat through, do not boil. Season to taste and finish with a pinch of nutmeg.