Meanwhile, melt 30g of the butter in a large frying pan over medium high heat. Add the leeks and cook, stirring occasionally, until softened, 6-8 minutes. Stir through the flour and cook for a further minute then gradually stir in the stock until smooth. Add the sour cream and nutmeg, season with salt and pepper and bring to a simmer and cook until thickened, 2-3 minutes.