In a large bowl, whisk together the flour, sugar, salt, and ½ teaspoon of the grated nutmeg. Add the cold, cubed butter and use a pastry cutter or your fingers to work it into the flour until the mixture resembles coarse crumbs. Add the egg yolk and ice water, mixing until the dough just comes together. Shape into a disk, wrap in plastic, and chill for 30 minutes.
Preheat your oven to 375°F (190°C). On a lightly floured surface, roll out the dough to fit a 9-inch tart pan. Press the dough into the bottom and sides, then trim the excess. Line with parchment paper and pie weights. Bake for 12 minutes, then remove the weights and bake for another 5 minutes until the base is dry and lightly golden. Let it cool slightly.
Lower the oven temperature to 325°F (165°C). In a medium bowl, whisk together the eggnog, three eggs, vanilla extract, and a pinch of salt until well combined and smooth. Avoid over-whisking to prevent too many air bubbles from forming in your custard.
Place the tart pan on a baking sheet to make it easier to handle. Carefully pour the eggnog mixture into the pre-baked crust. Sprinkle the remaining ½ teaspoon of nutmeg evenly over the top. Bake for 25 to 30 minutes, or until the edges are set but the center still has a slight jiggle when moved.
Remove the tart from the oven and let it cool completely to room temperature on a wire rack. Once cooled, transfer the tart to the refrigerator and chill for at least 3 to 4 hours, or ideally overnight. This ensures the custard sets properly for clean slices.