Soak potatoes in water. Cut the potatoes into strips about ¼-inch thick and 3-inches long, then soak in cold water for at least 30 minutes (or up to 12 hours) to remove excess starch. When ready to move on, drain and pat the fries completely dry with a towel.
Season the steak. Bring the steak to room temperature about 20 minutes prior to cooking. Season generously on both sides with kosher salt and black pepper.
Heat oil. Pour enough oil into a large, deep pot to come up about 3 or 4-inches. The pot should be no more than half full (or it will overflow when you add the potatoes). Heat over medium heat until the oil reaches 300°F (150°C).
First fry. Working in batches, carefully add a few handfuls of potatoes to the oil, lowering them gently. I find it helpful to use a spider skimmer to avoid splashing oil. Cook for 6-7 minutes, moving the fries around every 30 seconds or so to avoid sticking, until the potatoes are a bit tender. They will not brown during this first fry.
Drain the potatoes. Using a spider or slotted spoon, carefully transfer the potatoes to a paper towel lined wire rack to drain. Allow to rest for 20-30 minutes or up to a few hours in advance.
Sear the steak. Heat a large cast-iron skillet over medium-high heat. Add 2 Tbsp neutral oil. When just beginning to smoke, add the steak(s) and cook undisturbed for 4 minutes, or until a deep crust forms. Carefully flip the steak(s), add 2 Tbsp unsalted butter, 2 smashed garlic cloves, and 3 thyme sprigs, then cook another 3-4 minutes for medium-rare while basting with a spoon. Transfer to a plate, then rest for 10 minutes, then slice against the grain or serve whole.
Internal temperature guide: Note that the temperature will continue to rise as the steak rests, so I recommend removing before the desired doneness is reached. Once rested, the final temperature should read: 125°F (52°C) for rare, 130°F (54°C) for medium-rare, or 135°F (57°C) for medium.
Second fry. When ready, reheat the same frying oil to 350°F (177°C). Working in batches, carefully lower the fries into the hot oil. Cook for 3-4 minutes, or until golden brown and crispy. Transfer to the wire rack (preferably with fresh paper towels!) to drain, then season immediately with Kosher salt and additional flavoring, such as garlic or herbs, if using. If adding other ingredients to the fries, I recommend placing them all in a large bowl, then tossing with the add-ins for maximum coverage.
Plate, then serve! Divide the steak between plates, sprinkle with flaky sea salt, then serve with heaping mounds of French fries on the side.