01 -
Fill a large bowl with ice and water. Peel the potatoes and slice them lengthwise into ⅓-inch thick sticks. Alternatively, use a mandolin slicer with a ⅜-inch setting. Add the potatoes to the ice water and soak for 15 minutes. Drain and pat completely dry.
02 -
Attach a deep-fry thermometer to the side of a large, heavy pot. Add the peanut oil and heat over medium-high heat. Once the oil reaches 300°F, carefully add the potatoes in batches and cook until beginning to soften but not browned, 3-4 minutes. Transfer to a paper-towel-lined sheet pan.
03 -
Heat a large skillet over high heat. Pat the steak dry and season all over with 2 teaspoons salt. Add the olive oil. Once the oil is just smoking, add the steak and cook until medium-rare, about 4-6 minutes per side. Transfer to a cutting board and top with the butter. Let the steak rest for 5 to 10 minutes.
04 -
Increase the peanut oil temperature to 350°F. Working in batches, return the potatoes to the hot oil and cook until golden brown and crispy, about 5 minutes. Transfer the fries to the paper-towel-lined sheet pan. Season with remaining ½ teaspoon salt and pepper to taste.
05 -
Thinly slice the steak against the grain. Serve with the frites and garlic aioli on the side.