It takes some amount of work to turn these:
You will need a food processor of some sort. Obviously in the old days this stuff had to be hand-grated, but even using the supergraters mentioned in my Orange Cake post this would take forever and be dangerously eye-watering. He used his mother's Magimix to pulverise roughly chopped pieces of peeled root.
Having crumbed up your roots, you will need a saucepan and some creme fraiche. At this point any mixture of the two will be either too mild or mouth-scrapingly hot; the best way we think to get the flavour correct is to very gently cook the mixture, taste-testing as you go. Adding vinegar, a small amount of icing sugar and/or flour is also an option, although not one which I saw the application of in detail. The key thing is correct flavour balanced with correct texture.
Once done the sauce will keep in the fridge for a few weeks, slowly getting milder and milder. We've been eating it with cold cuts and smoked mackerel on toast. Dyed green it's also a cheap substitute for wasabi. Added to mustard it forms Tewksbury mustard.
Real horseradish is apparently very easy to grow in Britain; the leaves are also edible, although they have the typical popularity of tuber-top greens at the moment. The boy's second rapidly-grasped idea was to grow our own, although he would have to indulge in some guerilla colonisation of his mother's garden to make this happen. Maybe one day ... but for now, another seasonal jarred thing to Make and Do has been created which I'm sure will become a traditional part of our culinary year.