This recipe is really useful. It goes well with so many different things and is rich and delicious. I had it for lunch today over green beans and a poached egg. I tried Heston Blumenthal's method for making the egg and I'm impressed. Came out perfectly - the white soft but totally set and the yolk runny.
This sauce also works well over fried sprats (or tinned sardines), chicken, hard boiled eggs... what ever you have handy.
1 egg yolk
2 tbsp butter
1/4 tsp dijon mustard
1/2 tsp white wine vinegar
Put all the ingredients into a bowl over a pan of hot, steaming water. If you have a very thick bottomed pan you might just get away with making it straight in there (I have one that works) but it is much less likely to scramble over an indirect heat. Keep stirring while the butter melts and then until it begins to thicken. Serve immediatly.