This is so easy and so good. The sauce will keep in your fridge for a week and is delicious served with fish cakes, smoked fish and I also love it with grilled or roast beef.
Put the egg yolks, mustard, sugar and vinegar in a bowl. Whisk well and add the oil gradually in a slow and steady stream while whisking all the time. The sauce will emulsify and thicken quite easily. Add the horseradish and chopped herbs. Taste and correct seasoning. It is unlikely to need salt because of the large quantity of mustard.
Chill until needed.