Whisk together flour and salt. Scatter butter over the flour, and, using your fingers, quickly work in 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.

Dice potatoes 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.

Slice leeks into rings 1/8 inch thick and place in a large bowl filled with cold water. Swish and wash thoroughly to remove any grit which should settle to the bottom. Scoop leek pieces into a colander. Rinse and drain. If necessary, repeat.

Melt butter in a large skillet over medium heat. Add leeks and cook slowly until tender, about 30-40 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.

Preheat oven to 350°F.

On a lightly-floured work surface, roll one disk of crust to 10 inches in diameter. Place in a deep-dish 8-inch pie pan. Roll the other disk to 10 inches in diameter.

Spoon filling into the crust-lined pan. Cover with reserved crust, folding the edges under and crimping with a fork.

Cut several long slits in the top crust, then brush with reserved egg white.

Place pie on a rimmed baking sheet and bake until crust is golden brown and sauce is bubbling through the slits, about 60 minutes.