Mix the cream and crème fraîche in a small saucepan and bring the mixture to the boil. Thoroughly mix the mustard powder, sugar and salt in a bowl. Add the hot cream mixture together with the honey, little at a time, mixing until a thickish, smooth paste forms.
Lightly whisk the egg and add to the mustard mixture. Pour the mixture back to the saucepan and bring gently to the boil, stirring continually. As soon as you see the first bubbles bursting on the surface of the mixture, take the pan off the heat.
Let the mixture cool a bit, then mix in the vinegar. Sieve the mixture to remove any lumps of coagulated egg and spoon it in a small clean jar (or jars). Cover tightly with lid and store in refrigerator. Preferably consume the homemade mustard within one week. Serve with various meat dishes, like barbecued sausages, Christmas ham etc.