Mashed Potato and Turnip Gratin

Directions

1. But­ter an 8×11-inch bak­ing dish and set aside.
2. 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.
3. 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.
4. 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.
5. Trans­fer the mash to the pre­pared bak­ing dish, smooth­ing the top with a spat­ula. Sprin­kle the remain­ing cheese on top.
6. 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.