Mashed Potatoes with Celery Root and Mascarpone

3 1/4 pounds potatoes, peeled, cut into 2-inch pieces
1 1/2 pounds celery root (celeriac), peeled, cut into 1/2-inch pieces
3 whole peeled garlic cloves plus 1 tablespoon minced garlic
8 ounces mascarpone cheese, room temperature
1/2 cup (1 stick) butter

Place potatoes, celery root and whole garlic cloves in large pot. Add enough cold water to cover vegetables. Salt the water and bring to boil. Cover partially and boil until vegetables are very tender, about 40 minutes. Drain.
Transfer vegetables to large bowl. Using electric mixer, beat mixture until almost smooth. Add minced garlic, mascarpone and butter; beat until smooth. Season to taste with salt and pepper. (Can be prepared 3 hours ahead. Let stand at room temperature. Rewarm over medium-low heat, stirring frequently.)
(From: Bon Appetit Magazine, 1997. www.epicurious.com)