Peel the parsley roots and cut into large pieces. Peel the onion and chop finely. Briefly sweat the parsley root and onion cubes in the butter and add the thyme. After about 3 minutes, deglaze with the poultry stock and let the roots cook over low heat. When the vegetables are cooked, add the cream. Then puree finely with a hand blender. The puree becomes even finer when it is finally passed through a fine sieve.