Remove root end of celery root, then peel remaining and cut into 1-inch pieces. Peel and core parsnip, then cut into 1-inch pieces. Place celery root and parsnip in a steamer basket set over a pot of boiling water and cover pot. Steam vegetables until they are very tender, 15 to 20 minutes. Transfer vegetables to a food processor with cream (stock), butter (avocado), salt and pepper and pulse until very smooth, about 1 minute. Season with salt and pepper.