From Vegetable Soups from Deborah Madison’s Kitchen 

Ingredients
For the stock:
scrubbed peels from the celery root
1 cup chopped leek greens or onion
3 long celery ribs, chopped
1 carrot, chopped
Bouquet garni of 1 bay leaf, 3 parsley sprigs, pinch of ried thyme
sea salt

For the soup:
2 Tablespoons butter
1 onion or 2 fat leeks, thinly sliced
3 cups diced celery root (1 to 1 1/2 pounds scrubbed celery root)
1 cup diced celery ribs
1 large tart apple, i.e. Granny Smith, peeled and thinly sliced
1/2 cup diced potato
1 cup half and half or milk

For the garnish:
3/4 cup very thinly sliced celery heart
1/4 apple, thinly sliced and cut into matchsticks
5 small lovage leaves, slivered, or other green herb like parsley or chives
blue cheese, crumbled

Directions
Slice away the celery root skins thickly, with a knife, cutting in more towards the center of at the base where the root is more gnarled (the dirt gets swirled into the flesh a bit here.) Put the trimmings in a pot along with the rest of the stock ingredients and 1/2 teaspoon salt. Add 6 cups water, bring to a boil, then lower the heat and simmer, covered, for about 25 minutes.

Meanwhile, get the soup started. Melt the butter in a wide soup pot, then add the onion, celery root, apple, potato, and 1 teaspoon salt. Cook over medium heat, stirring occasionally, for about 10 minutes. Add 1/2 cup water, cover the pot, and continue cooking for another 10 minutes.

The stock should be just about finished cooking now. Strain it directly into the vegetables, bring to a boil and then lower the heat, cover, and simmer until the vegetables are soft, about 20 minutes. Puree the soup until smooth, then stir in the half-and-half or milk and taste for salt.

Divide the soup among bowls and top with the garnish ingredients.