Potato, Parsnip and Parmesan Gratin.

4 lb Yukon Gold potatoes, peeled and quartered

Kosher salt

1 lb Parsnips, peeled, quartered lengthwise, cored and cut into 2-inch pieces

4 T unsalted butter, cut into pieces

8 oz creme fraiche or yogurt

4 oz mascarpone

1/4 tsp freshly ground nutmeg

Freshly ground black pepper

1 egg white, whipped until lightly foamy

1 oz finely grated Parmigiano-Reggiano (about 1 cup)

Put potatoes and parsnips in a pot, cover with water and add 1 T salt. Bring to a boil and cook until the potatoes are tender, about 20 minutes. Drain the potatoes and parsnips, put them back into the pot with butter, and coarsely mash with a potato masher. Fold in the creme fraiche or yogurt, mascarpone, nutmeg, t tsp salt and 1/4 tsp pepper. Fold in egg white. Transfer to a 9x13-inch baking dish. Heat the oven to 375ºF. Sprinkle the gratin with the cheese, and bake until the gratin is heated through and the top is golden, about 40 minutes.

Serves 8 to 10