Preparation

• Put the potatoes and parsnips in a 6- to 8-quart pot, cover by about 3 inches with cold water, add 1 Tbs. salt, bring to a boil, and cook until the potatoes are easily pierced with a fork, about 20 minutes after the water begins to boil. Drain the potatoes and parsnips, put them back into the pot with the butter, and coarsely mash with a potato masher. Fold in the crème fraîche, mascarpone, nutmeg, 2 tsp. salt, and 1/4 tsp. pepper. (If you prefer a smoother texture, whip with a hand mixer on medium high until light and fluffy, about 1 minute.) Season to taste with salt and pepper and then fold in the egg white. Transfer to a 9×13-inch baking dish.

• Position a rack in the center of the oven and heat the oven to 375°F. Remove the plastic, sprinkle with the cheese, and bake until the gratin is heated through and the top is golden, about 40 minutes. Let cool for a few minutes before serving.

Make Ahead Tips

The assembled gratin can be refrigerated for up to 3 days (minus the Parmigiano sprinkled on top). Let cool completely, wrap well in plastic wrap, and refrigerate. Take the gratin out of the refrigerator about an hour before you plan to bake it.

The assembled gratin can also be frozen for up to 3 weeks. the texture becomes a little grainy but nothing you’d notice once topped with gravy. Cool completely, wrap in well in plastic, and freeze. Thaw in the refrigerator overnight, then let it sit at room temperature for 1 hour before sprinkling with Parmigiano and baking.