I know what you are thinking…TURNIPS???…. but just try this recipe and you will have a whole new outlook on how to use turnips. I served this dish at a dinner party and my friends said they were the best mashed potatoes they have ever eaten. The turnips give a subtle, fresh flavor. This is more of a mashed potato dish than a gratin. Please double the recipe if you have more than 6 guests for dinner.
Unsalted butter, softened, will need more for the baking dish
Medium potatoes, preferably Yukon Gold, peeled and cut into 1 1/2 chunks- really any type of potato would work
Small turnip, peeled and cut into 1 inch chunks
Cream cheese softened and cut into chunks
Grated parmesan cheese
Whole milk as needed or use whatever milk you have on hand
Put the potatoes and turnip in a 5– to 6-quart pot or Dutch oven and cover with cool water by about 1 inch (the turnip will float to the top). Add 1/2 tsp. salt and bring to a boil over medium-high heat. Reduce the heat to medium, cover partially, and cook until the potatoes are very tender and the turnip is soft, about 25 minutes.
Drain the vegetables and return them to the pot. Set over medium heat for a minute or two, shaking and stirring to rid them of excess moisture and steam.
Transfer the vegetables to a stand mixer fitted with the paddle attachment (or use a large mixing bowl and an electric hand mixer). On low speed, break up the vegetables until the potatoes are smooth and there are small chunks of turnip in the mixture. Gradually add the cream cheese, increase the speed to medium, and beat until absorbed. Gradually add the butter and beat until absorbed. Add the mustard and half of the parmesan and beat until combined. If the purée seems too dry, add enough milk to make it light and fluffy but not wet. (You may not need to use any milk.) Season to taste with salt and pepper.
Transfer the mash to the prepared baking dish, smoothing the top with a spatula. Sprinkle the remaining cheese on top.
Position a rack in the center of the oven and heat the oven to 375°F. If the gratin was made assembled ahead and refrigerated, let it sit at room temperature while the oven heats. Bake uncovered until heated though and the top is golden, about 40 minutes for a refrigerated gratin or about 20 minutes for a freshly made gratin. Let sit for 10 minutes before serving.
The gratin may be made 2 days ahead, up to the point of baking. Cover it with plastic wrap and refrigerate it.