Root Vegetable Purée

In a large saucepan cover rutabagas and carrots with salted water. Bring water to a boil and simmer, covered, until vegetables are very tender, 40 to 50 minutes. While rutabagas and carrots are simmering, peel potatoes and cut into 1/4-inch pieces. In another large saucepan cover potatoes with salted water. Bring water to a boil and simmer potatoes, covered, until tender, 15 to 20 minutes. Drain rutabagas and carrots well in a large sieve and in a food processor puree until smooth. Drain potatoes well in sieve and force through a ricer or food mill fitted with a medium disk into a large bowl. Stir in rutabaga-carrot puree, butter, and salt and pepper to taste and combine well. Puree may be 3 days ahead and chilled, covered. Reheat puree before serving.