Preheat oven to 180C. Using the coarse grater disk on a food processor or the largest holes on a box grater, shred the potatoes. Toss with a pinch of salt and a good grinding of black pepper into a large bowl. Transfer to a clean dishtowel, gather together the ends and thoroughly wring out the excess liquid over the sink. Transfer potatoes to a bowl and set aside.
Heat oil and 2 tbsp. butter in a 10" cast-iron pan over a medium-high heat until butter is melted. Add potatoes and immediately start forming into a crust by pushing potatoes flat against the bottom and sides of the pan with a fork. Continue cooking, pressing potatoes up sides of pan if they start shrinking, until potatoes are bound together and bottom of crust is starting to brown, about 10 minutes. Remove pan from heat and set aside.
Meanwhile, melt remaining butter in a small pan over a medium heat. Add the lardons and fry for 5-6 minutes; set aside.
Whisk 2/3 of the gruyère, crème fraîche, cream, eggs and a pinch of nutmeg in a large bowl.
Sprinkle lardons evenly over bottom of crust, then pour in egg mixture. Sprinkle over the remaining gruyère and bake until quiche is set and the crust is well browned - about 30-35 minutes. Let cool to room temperature before cutting into wedges and serving from the pan.
