Peel celeriac and onion, chop finely and sauté in 1 tbsp. butter. Pour in broth and cream and simmer for 20 minutes.
Chop the green of celeriac. Clean and peel celeriac and carrots. Cut carrots into sticks and celeriac into slices. Boil in salted water firm to the bite. Strain, rinse with cold water and let drain.
Puree soup, season with salt, pepper, nutmeg and lemon juice. Stir in celeriac green and vegetable.