I got hooked on this when stationed in Germany, but I never see it in the shops, and as I do not like vinegar the original French version just tastes yeuk.
1 nice egg
Oil of choice - I prefer sunflower.
1 level tsp of mustard powder
1/4 tsp ground white pepper
1/4 tsp sugar
salt to taste
Put everything but the oil and salt in a whizzer thingy jar and start to blitz. Slowly pour in the oil until no more can be incorporated - the resulting emulsion will begin to form a sheen. Add salt as required. That's it: done. It makes about 1/3 of a small jar and will keep in the fridge for four to five days in a closed container.
It goes wonderfully with chips and makes the best egg mayo or coleslaw ever.
To make an even richer and more unhealthy version use two egg yolks and do something else with the whites.