Fries method adapted from here.
Preheat an oven to 250°F. Season steaks generously with kosher salt and black pepper, then place on a baking sheet fitted with a rack. Roast in the oven until an instant-read thermometer reads 125°F for medium rare (45 - 55 minutes). Let rest for 5 minutes.
Preheat a cast iron skillet over medium-high heat while the steaks rests. The skillet should be smoking hot. Sear the steaks in the hot skillet until deeply golden on both sides (about 45 seconds per side). Let rest for 10 minutes.
Meanwhile, prepare the fries and frizzle leeks. Using a mandoline slicer, carefully cut potatoes into fries and julienne leek lengthwise.
Place fries into a large dutch oven (4 to 5 quart), then cover with oil by about an inch.* Place pot over high heat, bring oil to a boil and cook, without stirring, for 15 minutes. At this point, you can use a slotted spoon to stir the fries and gently release any that may be stuck to the bottom. Continue cooking until the fries are crispy and golden (5 to 10 minutes more). Transfer fries to a paper towel-lined baking sheet and immediately season well with salt.
Carefully drop the sliced leeks into the hot oil, and cook until golden and crispy (2 to 3 minutes). Transfer leeks to the same lined baking sheet and immediately season with salt.
To serve, plate up each steak with a serving of fries and top with frizzle leaks. Serve plenty of truffle aioli on the side for dipping.
* You can certainly cook french fries using whatever method you are most comfortable with. Just don't plan on using the oven, because the steaks will be cooking in the oven while you are preparing the fries. An airfyer would be a great option here!