This sauce goes well with a piece of roast beef, but can also be served with other roasts. You make this sauce after you have prepared the meat. Remove all but 3 tablespoons of the (frying) butter from the pan in which the roast beef or roast is roasted. Fry the chopped onion in the (frying) butter for 1 minute, stirring constantly. Then add the garlic and parsley. Stir everything together and carefully sprinkle the flour through a sieve in small quantities. Mix everything well again and pour in the stock little by little while stirring constantly. Now keep stirring until you get a thickened sauce. Reduce the heat and let the sauce simmer gently for 8 to 10 minutes. Pour the sauce through a sieve into a small saucepan. Bring the sauce to the boil again. Mix the Marne French Mustard with the cream, which is at room temperature. Add 3 tablespoons of the mustard cream to the sauce and stir well. Add some pepper to your own taste. Do not let the sauce simmer for too long. The rich mustard flavor would otherwise evaporate too much due to heating. If you find the sauce just too spicy, you can cook the sauce a little further. If you like more mustard flavor, heat the sauce for a shorter period of time. Just before serving, stir the chives or spring onion into the sauce. Serve the sauce in a preheated sauce boat.