Put the potatoes in a pan of cold salted water and slowly bring to the boil, then reduce the heat and simmer for 5 minutes. Turn off the heat and leave the potatoes in the hot water for 15 minutes so that they are half cooked. Drain and peel.
Coarsely grate the potatoes, beetroot and onion into a large bowl. Finely grate the horseradish into the same bowl.
Bash the garlic into a