If you're lucky enough to have a horseradish plant, you'll need to pull a decent-sized root from the ground - this takes a little patience! Or buy a fresh horseradish root. Clean it, peel it, and dice or grate it very finely. Place it into a small blender.
Add in the creme fraiche, vinegar and mustard, with some seasoning, and blend until relatively smooth. Very small visible lumps are fine.
Chill in the fridge for at least an hour for the flavours to develop.
The creamed horseradish sauce will keep in a tightly sealed jar in the fridge for 2-3 weeks.
Notes
The fresh creamed horseradish sauce will keep well for 2-3 weeks (sometimes longer). But always trust your judgement and look and smell before you eat it - or throw it away.