Peel the carrots, grate roughly. Cut away the peel from the celeriac, also grate it roughly.
Drizzle with half of the lemon juice. Peel and roughly grate the apple and drizzle with the remaining lemon juice. Mix everything well. Roughly chop the walnuts and add to the salad.
Make a dressing from yogurt, sugar, salt and pepper and pour over the salad. Mix everything together well and let it steep for at least 1 hour.