2 Tbs. unsalted butter
6 medium turnips, about 1½ lbs.
4 medium Yukon Gold potatoes (about 1½ lbs.), peeled
4 medium rutabagas, about 1½ lbs.
6 cups heavy cream
3½ tsp. kosher salt
1/2 heaping tsp. cracked black pepper
Leaves of fresh sage, optional
Preheat the oven to 325°F. Butter a large rectangular baking dish (10¼ x 7½ x 2¾ inches recommended).
Thoroughly wash the vegetables. Peel the turnips and rutabagas if their skin is thick and fibrous. Slice the vegetables approximately 1/4-inch thick, placing the slices in a bowl of cold water to prevent oxidation. Once all have been sliced, drain them and place in a medium stockpot. Combine the cream, salt, and pepper and add it to the pot. The cream should completely cover the vegetables.
Bring the pot to a simmer over low heat. Continue simmering until the vegetables are cooked about halfway through, approximately five to seven minutes.
Strain the vegetables, reserving the cream to finish the gratin. Spread the vegetables out in the baking dish. Pour enough cream over the vegetables to just barely cover them; you may not need all of it. (Any leftover cream can be used for the soup in the following recipe.) Bake the gratin for 45 minutes, or until the top is firm and golden brown. Allow to cool at room temperature for at least 15 minutes before serving. Scatter sage on top if desired.