Prepare the beets: wash them, wrap in aluminum foil and bake for an hour in 180°C. Then let them cool down, peel gently and grate to very tiny shreds.
Peel the horseradish and grate it to the same size tiny shreds.
Set the shredded beets, horseradish and all the other ingredients in a bowl. Mix gently and season with more salt and/or black pepper if necessary.