Prepare pie dough. Chill at least one hour or up to two days. Roll out and cut six five-inch circles to press into jars or to line and crimp bottom crust for nine-inch pie. Reserve dough for top crust(s). Refrigerate pie shells until ready to use.
Preheat oven to 400 degrees. Toss potatoes with olive oil, salt and pepper. Spread on large baking sheet and roast until golden brown, about 30 minutes, stirring halfway through.
Turn oven down to 375 degrees.
Saute leeks and garlic in butter with a pinch of salt and pepper til slightly soft and leeks turn bright green. In a large bowl, combine potatoes, leek mixture, cheese, cream and mustard and mix all together.
Spoon into pie crusts, cover with top crusts and crimp edges to seal. Cut air vents in top crust and brush with egg. Arrange mason jars on large baking sheet and place in center of oven. Bake until golden brown, about 45 minutes, rotating halfway through.
This recipe is provided courtesy of Tiny Pies in Austin.