Bring 1 inch of water to a simmer in a large pan or Dutch oven. Place celery root, rutabaga and potatoes in a large steamer basket over the water, cover and steam over medium-low heat for 20 minutes. Add garlic and continue steaming--checking the water level and replenishing as necessary--until the vegetables are fall-apart tender, 20 minutes more.

Remove the vegetables, drain the cooking liquid and return the vegetables to the pan. Add 2 tablespoons butter and mash until chunky-smooth. Gradually stir in buttermilk, salt, pepper and nutmeg.

Just before serving, stir in the remaining 2 tablespoons butter and chives.

Tips: To peel celery root and rutabaga, cut off one end to create a flat surface to keep it steady. Cut off the skin with your knife, following the contour of the root. Or use a vegetable peeler and peel around the root at least three times to ensure all the fibrous skin has been removed.

No buttermilk? You can use buttermilk powder prepared according to package directions. Or make "sour milk": mix 1 tablespoon lemon juice or vinegar to 1 cup milk.

MAKE AHEAD TIP: Prepare through Step 2 and refrigerate for up to 2 days. Reheat in a double boiler and stir in the remaining butter and chives (Step 3) just before serving.