GNOCCHI: Boil potatoes until tender, approximately 20 minutes. Peel, mash, and run through ricer or fine-mesh sieve to remove lumps. Refrigerate, uncovered, overnight. (This removes moisture from potatoes for light, fluffy gnocchi.)

Combine flour, potatoes, and salt in a large bowl. Make a well in center, and add egg yolks. Incorporate yolks into potato-flour mixture, a little at a time, until completely combined. Knead by hand until a smooth dough forms. (If it’s too sticky, add more flour.) Place dough on a well-floured surface and shape into a ball. With a sharp knife, divide dough into quarters. Working with one dough section at a time, roll into a ½-inch-diameter tube. Cut into 1-inch pieces, and set on a baking tray lined with parchment paper. Cover, and repeat with remaining dough.

Cover gnocchi and refrigerate. Meanwhile, bring a pot of well-salted water to a boil. Cook gnocchi a dozen at a time. When they float, let cook 90 seconds more, and then remove with a slotted spoon.

CASSEROLE: Preheat oven to 350°F. Combine Gruyère and ¾ cup raclette. In a lightly oiled 2-quart casserole dish, alternate layers of gnocchi and Gruyère-raclette mixture, ending with a layer of cheese. Sprinkle sage on top.

In a small pan, bring half and half to a simmer. In a small bowl, beat reserved egg yolk, then whisk in half and half and a pinch of white pepper. Pour custard evenly over gnocchi and cheese layers. Top with remaining ¾ cup raclette. Bake 30 minutes. Do not overcook, or custard may break. Let cool 5 to 10 minutes before serving.