Potato and Leek Gratin

Separate white and light green parts of leeks from dark green parts. (Save dark green parts for another use.) Cut white and light green parts in half lengthwise, then cut crosswise into 1/2-inch-thick slices.

Melt butter in a large deep-sided skillet over medium. Add leeks and 3/4 teaspoon salt; cook, stirring occasionally, until leeks are softened and starting to brown on edges, 10 to 12 minutes. Add garlic and chopped thyme; cook, stirring, until fragrant, about 1 minute.

Stir cream, mustard, pepper, nutmeg, paprika, and remaining 11/2 teaspoons salt into skillet. Bring to a simmer over medium, stirring occasionally. Let simmer for 1 minute. Remove from heat and stir in cheese until melted.

Preheat oven to 350°F. Coat a 2- to 3-quart baking dish with butter.

Peel potatoes and slice into 1/16- to 1/8-inch-thick pieces. Add sliced potatoes directly to cream mixture as you cut, gently stirring to ensure each slice is coated with cream mixture on both sides and no slices are stuck together.

Transfer mixture to prepared baking dish. Shake dish to help potatoes settle into an even layer. Cover dish tightly with aluminum foil.

Bake until potatoes are starting to become tender, 45 to 50 minutes. Uncover and continue baking until potatoes are tender and top is golden, 25 to 35 minutes. Let rest for 10 minutes. Garnish with thyme leaves. Make-Ahead and Storage Instructions Gratin can be assembled (without baking) up to 1 day ahead and stored tightly covered in the refrigerator. If baking from cold, allow 10 to 15 minutes extra for baking. Leftovers can be stored in an airtight container in the refrigerator for up to 3 days.