Gnocchi Gratin

1 pound russet potatoes
1 large egg
1 large egg yolk
6 ounces Parmesan, finely grated, divided
1 cup all-purpose flour plus more
1 teaspoon kosher salt plus more
1/2 teaspoon freshly grated nutmeg
1 cup heavy cream
1 sprig rosemary
1 1/4″-thick slice fresh ginger
2 tablespoons (1/4 stick) unsalted butter, room temperature, divided
3 tablespoons fine dried breadcrumbs
4 ounces Gorgonzola dolce or other mild blue cheese

Place potatoes in a large pot. Add water to cover by 1”; bring to a boil, reduce heat to medium, and simmer until tender, 25–30 minutes. Drain; set aside and let cool slightly.
Peel potatoes. Pass through ricer or a food mill or sieve, into a large bowl. Whisk egg and egg yolk in a small bowl to blend. Add egg mixture, 1/3 cup Parmesan, 1 cup flour, 1 tsp. salt, and nutmeg to potatoes. Using your hands, gently mix to form a soft but not sticky dough. Transfer dough to a floured surface. Gather into a ball, then divide into 4 pieces.

Line a rimmed baking sheet with parchment paper; dust with flour. Working with 1 piece at a time (keep other pieces covered with a damp kitchen towel), roll dough into a 24”-long rope. Cut crosswise into 1” pieces. Working with 1 piece at a time, press lightly on gnocchi with the back of the tines of a fork and gently roll gnocchi to create ridges on 1 side. Place on prepared baking sheet and lightly dust with flour. DO AHEAD: Can be made 4 hours ahead. Cover and chill. Cover and chill remaining grated cheese.

Bring cream, rosemary, and ginger to a boil in a small saucepan. Remove from heat; cover and let steep for 30 minutes. Set aside.

Preheat oven to 375°. Bring a large pot of salted water to a boil. Working in batches of about 12 at a time, cook gnocchi until they float to the surface, 2–3 minutes. Using a slotted spoon, transfer gnocchi to a baking dish coated with 1 Tbsp. butter.

Mix 3 Tbsp. Parmesan, remaining 1 Tbsp. butter, and breadcrumbs in a small bowl. Strain reserved cream mixture into dish all over gnocchi. Break off small pieces of Gorgonzola; scatter over. Sprinkle the remaining Parmesan over, then the breadcrumb mixture.

Bake until the filling is bubbling and the topping is browned, 25–30 minutes.

Source: Bon Appetit