Potato, Leek and Celery Root Soup

This recipe is from Annie Somerville's Fields of Greens, with only minor changes. I discovered it when I was looking for something to do with the celery root (celeriac) that I bought at the Winter Farmers' Market. Now I make this soup all the time because it's so yummy! In fact, we're making it part of our Christmas dinner this year.

6 cups vegetable stock
1 medium size celery root bulb, about 1 lb.
2 lbs. large red or yellow finn potatoes, peeled and thinly slices, about 7 cups
salt
white pepper
1 bay leaf
4 garlic cloves, finely chopped
1 Tbsp olive oil
1 Tbsp butter
2-4 medium sized leeks, cut in half lengthwise, thinly sliced and washed (3-4 cups)
1/4 cup dry white wine

Peel, quarter and thinly slice the celery root, discarding the inner core if it's soft and spongy.

Place the potatoes and celery root in a soup pot with 1 quart stock, 1 tsp. salt,
a few pinches of white pepper, the bay leaf and the garlic. Bring to a boil, then reduce the heat, cover the pot, and simmer for 30 minutes, until the potatoes are very soft. Remove the bay leaf, then puree with an immersion blender (or pass through a food mill, mash with a potato masher, or otherwise puree). Return to the pot (where necessary), add remaining stock, cover, and cook over low heat.

While potatoes and celery root are cooking, heat the olive oil and butter in a sauté pan and add the leeks, 1/2 tsp salt and a few pinches of pepper. Sauté over medium heat until the leeks begin to soften, about 3-4 minutes. Then cover the pan and lightly steam them for 5-10 minutes. Add the wine and simmer, uncovered, until the pan in almost dry.

Add the leeks to the potatoes, cover, and cook over low heat for 20-30 minutes. Season with salt and pepper to taste.