How to Make It
Preheat oven to 350°F. Place potatoes on an aluminum foil-lined baking sheet. Bake in preheated oven until very tender when pierced with a knife, about 1 1⁄2 hours. Cool 30 minutes. Peel and discard potato skins.
Stir together potatoes, shallot, milk, eggs, sage, nutmeg, 1⁄2 cup of the melted butter, 2 teaspoons of the salt, and 1 teaspoon of the pepper with a fork until mostly smooth. Pour mixture into a lightly greased 13- x 9-inch baking dish.
Combine pecans, breadcrumbs, cheese, and remaining 1⁄2 cup butter, 1⁄2 teaspoon salt, and 1⁄4 teaspoon pepper in a medium bowl. Sprinkle pecan mixture in an even layer over potato mixture.
Bake, uncovered, at 350°F until topping is golden brown, 28 to 30 minutes. Sprinkle with parsley.
To make ahead, prepare recipe through Step 3. Cover and chill overnight. Let baking dish sit at room temperature while oven preheats. Proceed with recipe as directed in Step 4, increasing bake time to about 45 minutes.