If desired, peel the potatoes. Slice them into even 1/2-inch sticks and place them into a large ice bath. Soak the potatoes for at least 30 minutes, then drain the water and pat them dry. Set the steak(s) out so it can come up to room temperature while the potatoes soak.
Heat about 6-10 cups oil in a dutch oven (or deep-sided pan) to 300 degrees F. Prepare a drying station for the fries once they come out of the oil by lining a baking sheet with paper towels and placing a wire rack over the towels.
When the oil is ready, add a batch of fries to the pot. (It can be full but not overcrowded). Cook until they are just starting to become golden, about 5-7 minutes. The timing will vary depending on how much your oil drops in temperature - expect it to drop at least 20-30 degrees. Scoop the fries out with a spider strainer, place them on the drying rack, and repeat until all the fries have been fried once. Let the first batch of fries rest for at least 10-15 minutes to cool slightly. Leave the oil over the heat.
While the fries are resting, season the steak with salt on both sides. Pat the steak dry and then season both sides with salt again.
Place a cast iron pan over medium-high heat. Melt 2 tablespoons of butter in the pan. Once hot, cook and sear the steak for about 4-5 minutes. Then flip the steak and lower the heat to medium (or medium-low, depending on your stove/pan). Cook for another 4-5 minutes or until cooked to your desired doneness.
While the steak cooks, check that the oil temperature has come back up to 300 degrees F. Cook the fries in the oil again, working in batches, for about 2-3 minutes or until your desired crispness is reached. Then, transfer them to the wire rack and season with salt. Repeat until all the fries are cooked.
Transfer the steak to a platter or cutting board and top with the remaining butter, if desired. Allow the steak to rest for 10 minutes while the fries finish cooking. Then slice the steak into strips, divide it between two plates, and serve with the fries!