Celery Root Bisque

1/2 stick of butter
1 1/2 pounds of celery root peeled and cut into 1/2-inch cubes (4 cups)
1/2 pound shallots, thinly sliced
1 1/2 teaspoons salt
Freshly ground black pepper
8 - 10 cups vegetable or chicken stock
1 T fresh lemon juice, or to taste
1 cup yogurt (optional)

Melt the butter in an 8-quart heavy pot over moderately high heat. Add celery root, shallots, salt and pepper to taste and cook, stirring occasionally, until vegetables are golden, about 12 minutes. (Note: if you don't have shallots, onions or leeks will work.) Add the stock, bring to a boil, and simmer uncovered until the celery root is very tender, about 15 minutes.

Puree the mixture in batches in a blender (or use a hand-held puree wand) until very smooth and transfer back to the soup pot. Stir in the lemon juice and, if necessary, additional water to thin to desired consistency. Season with salt and pepper and keep the soup warm, covered. Whisk in the yogurt and serve.