For the steak: In a small bowl, combine the black pepper, celery salt, sugar and cayenne. Rub the mixture all over the steak. Drizzle the steak with the olive oil, then rub again to coat the steak. Wrap in plastic and let marinate in the refrigerator for at least 2 hours or up to overnight. Take the steak out of the fridge 1 hour before cooking.
When you are ready to cook the steak, heat a large cast-iron pan or heavy skillet over medium-high heat. Add the steak and cook, only turning once, until the internal temperature reads 125 degrees F for medium rare, 5 to 6 minutes on each side (be sure to brown the ends as well). Take the steak out of the pan and let rest on the cutting board for 10 minutes. Finish with the butter melted in the pan with a squeeze of lemon juice and spooned over the steak.
Slice the steak against the grain and serve with the frites.
For the frites: Wash the potatoes. Cut into 1/8-inch batons, then pat very dry with paper towels.
For the frying, fill a large Dutch oven with about 5 inches avocado oil. Use a candy thermometer to bring the oil to 350 degrees F. Fry the potatoes in small batches, so that the pot is not overcrowded, until golden brown and crispy, about 5 minutes. Drain well on a wire rack or a paper towel-lined sheet tray and season with the sea salt while they are still warm.
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