Preheat oven to 175°C.
Peel and dice the beetroot, celery root and potatoes. Place each in separate baking dishes and toss each with 2 tablespoons of oil and season lightly.
Cover each dish with foil and bake until each is tender (celery root takes about 30 minutes, potatoes about 40 minutes and beets 50 minutes). Let cool.
Whisk vinegar, horseradish, mustard, dill, mayonnaise and sour cream together. Stir in cooled vegetables and season to taste.
Salad is best prepared a day ahead and stirred a couple of times while chilling (the rosy beetroot colour will permeate the salad).