Preheat oven to 375 degrees. Add the flour, butter, and salt to a food processor. Pulse until you get the texture of fine breadcrumbs. With the processor running, slowly drizzle in enough cold water until it forms a ball, and immediately stop (you may not use all of the water). Remove the dough and knead a little on a floured board until smooth. Wrap in plastic wrap and refrigerate 30 minutes.
While the dough is resting in the refrigerator, add the potato slices to a large pan of boiling water and cook for 2 minutes, drain, and lay out onto a towel to dry.
Remove the pastry from the refrigerator and roll out onto a floured surface, one inch larger than the quiche pan. Roll the pastry over the rolling pin, lift, and place into a 9-inch round tart pan pressing the bottom firmly and the sides allowing the pastry to hang over the sides, do not trim. Do not stretch dough or it will shrink in the oven. Poke holes all over the bottom of the pastry using a fork. Lay a sheet of parchment paper over the pastry and fill the center with dried beans or uncooked rice. Bake the crust for 10 minutes.
While the crust bakes, add the eggs, milk, nutmeg, cheese, salt, pepper, fennel, and leek to a bowl; whisk to combine.
Remove the crust from the oven and remove the parchment paper and beans. Trim the excess crust from over the edges to make it look pretty. Return the crust back to the oven, uncovered, for 5 more minutes until it starts to brown. Remove from the oven and allow the crust to cool for a couple of minutes.
Arrange the potato slices inside the pastry, saving 40 slices for the top, and pour in the egg mixture so it comes almost to the top of the pastry edge. Arrange more potato slices on top. Bake for approx. 25–30 minutes until the eggs are set.
Allow the quiche to sit for 15 minutes before slicing.