Makes 8 servings
Make Ahead: The celery root can be prepared up to 8 hours before cooking.

Celery root puree may look like mashed potatoes, but when my guests take their first bite, their faces light up with surprise. More people should know about this delicious vegetable, which is at its peak during the late autumn. The size varies, so you could need anywhere from two large roots to four smaller ones for this dish. It isn't the prettiest vegetable at the produce store--my market now sells celery root pared of its dirt-crusted, gnarly brown skin, just in case it might offend someone. I suppose. If your celery root comes au naturel, pare the celery root first before rinsing, or you'll end up with mud all over your kitchen counter.

3 pounds celery root
1 pound baking potatoes, peeled and cut into 1 inch chunks (if preparing ahead, store in a bowl of cold water for up to 8 hours)
4 tablespoon unsalted butter
1/2 cup milk, heated
Salt and freshly ground pepper, preferably white pepper, to taste

1. Using a sharp knife, pare off the skin and gnarly roots from the celery root. Rinse well. Cut the celery root into 1 inch cubes, paring away any soft woody spots. (The celery root can be stored in a bowl of cold water at room temperature for up to 8 hours. Drain well.)
2. Place the celery root and potatoes in a large saucepan and add enough lightly salted cold water to cover the potatoes by 1 inch. Cover and bring to a boil over high heat. Reduce the heat to medium-low and set the lid ajar. Cook until the potatoes are tender when pierced with the tip of a sharp knife, about 25 minutes.
3. Drain the celery root and potatoes well and return to the pot. Stir the vegetables over medium-low heat until they begin to film the bottom of the pot, about 2 minutes. Remove from the heat.
4. Add the butter to the potatoes. Using a potato masher or a hand-held electric mixer, mash the potatoes, adding enough of the hot milk to reach the desired consistency. (The celery root mixture can also be pureed in a food processor. Don't try this with mashed potatoes--they are too starchy and will turn into a gluey mess.) Season with the salt and pepper. Transfer to a serving dish and serve hot.