Preheat oven to 350F (180C).
Peel and dice beets, celery root and potatoes, and place each in 3 separate baking dishes. Toss each with 2 Tbsp 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. Stir in cooled vegetables and season to taste.
Potato salad is best prepared a day ahead and stirred a couple of times while chilling (the rosy beet colour will permeate the salad).
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