Double Celery Soup

From epicurious.com!

Ingredients
2 tablespoons (1/4 stick) butter
1 tablespoon olive oil
2 large leeks (white and pale green parts only), thinly sliced (about 2 cups)
1 large onion, chopped
2 medium-size Yukon Gold potatoes (about 12 ounces), peeled, cut into 1-inch cubes
2 medium celery roots (celeriac; about 1 1/2 pounds total), peeled, cut into 1/2-inch cubes
2 large fresh thyme sprigs
1 bay leaf
8 cups low-salt chicken broth\
1/2 cup heavy cream
5 celery stalks with leaves, stalks thinly sliced, leaves reserved

Prep:

Melt butter with oil in heavy large pot over medium heat. Add leeks and onion and sauté until almost tender, about 10 minutes. Stir in potatoes, celery roots, thyme, and bay leaf. Add broth and bring to boil for a few minutes. Reduce heat, cover, and simmer until vegetables are tender, about 40 minutes. Add celery stalks and simmer until all vegetables are very tender, about 12 minutes longer.
Puree soup in pot. Stir cream into soup. Season to taste with salt and pepper.  Garnish with reserved celery leaves and serve.