1. Melt the butter and stir in the flour off the heat. Slowly add the water, stirring constantly to make a smooth paste. Season well and return to the heat.
2. Add the dill, vinegar and sugar, and cook on a gentle heat for 5 minutes. Stir in the yoghurt or sour cream and warm through gently.
3. A delicious sauce which can dress up plain chops or steak. It is also excellent with poached fish.