Whisk together flour and salt. Scatter butter over the flour, and, using your fingers, quickly work into the flour until the mixture resembles coarse crumbs.

Add egg yolk and 1 Tbs water and mix together to form a dough. Add additional cold water, if needed.

Divide dough in two and shape into disks. Wrap each in plastic and refrigerate at least 3 hours, preferably overnight.

Reserve egg white.

Peel and slice potatoes 1/8 inch thick and place in a saucepan filled with salted water over medium heat. Bring to a boil, reduce heat, and simmer until tender, about 20 minutes. Drain, reserving 1/2 cup of the cooking water.

Melt butter in a large skillet over medium heat. Add onions and garlic and cook slowly until tender, about 10 minutes. Stir in flour, followed by milk, stirring until thickened and bubbling. Add cheese and stir until melted. Remove from heat and stir in potatoes. If needed, stir in several tablespoons of cooking water to make a sauce.

Season with nutmeg, and salt and pepper to taste. Set aside.

On a lightly-floured work surface, roll one disk of crust to 10 inches in diameter. Place in a 9-inch pie pan. Pierce the bottom with a fork and freeze for at least 20 minutes.

Preheat oven to 350°F.

Brush pie crust with reserved egg white.

Spoon filling into the crust-lined pan and bake until crust is golden brown and sauce is bubbling, about 45 to 60 minutes.