Alsatian Potato Pie

Name: Alsatian Potato Pie
Ingredients: 3 Yukon gold potatoes (about 1 ½ pounds), peeled and cut into ¼-inch-thick rounds; Kosher salt and freshly ground pepper; 1 cup heavy cream; 5 cloves garlic, crushed with the flat side of a large knife; ½ teaspoon freshly grated nutmeg; 2 tablespoons unsalted butter; 1 medium leek, white and light-green parts only, halved lengthwise, thinly sliced crosswise and well washed; ¼ cup chopped fresh flat-leaf parsley, plus a few small sprigs for topping (optional); 1 large egg yolk; All-purpose flour, for work surface; 1 sheet frozen all-butter puff pastry (about 14 ounces), such as Dufour, thawed; 1 ½ cups grated Comté or Gruyére