In a Dutch oven, place potatoes and add enough oil to cover by an inch. Be sure the oil line is at least 2 inches from the top edge of pot. Cook potatoes over medium-high to high heat and bring oil to a boil. Cook 20-30 minutes, stirring frequently, or until frites are deep gold. Remove with a slotted spoon to a cooling rack set over a baking sheet to drain. Sprinkle with flaky sea salt to taste.
While the potatoes are cooking, season the steak liberally with salt and pepper. Heat a cast-iron skillet over medium-high heat. Rub skillet with oil and drizzle some over steaks. Cook steaks to deep and crusty, and browned on both sides, about 8 minutes for rare and 10 minutes for medium. Let steaks rest.