1. Fill a large bowl with ice and water. Peel the potatoes and slice them lengthwise into ⅓ -inch thick sticks. Alternatively you can use a mandolin slicer that has a ⅜-inch setting. Q!Zwedr  njk[Add the potatoes to the ice water and soak for 15 minutes. Drain and pat completely dry.
2. Attach a deep-fry thermometer to the side of a large, heavy pot. Add the peanut oil to the pot over medium-high heat. Once the oil reaches 300°F, working in batches, carefully add the potatoes to the hot oil and cook until beginning to soften but not brown, 3 to 4 minutes. Transfer the potatoes to a paper-towel-lined sheet pan.
3. Meanwhile, heat a large skillet over high heat. Pat the steak dry and season all over with the salt. Add the olive oil to the skillet. Once the oil is just smoking, add the steak and cook until medium-rare (see Note) about 4 to 6 minutes per side. Transfer the steak to a cutting board and top with the butter. Let the steak rest for 5 to 10 minutes. 
4. 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 salt and pepper to taste.  
5. Thinly slice the steak against the grain. Serve with the frites and garlic aioli alongside. 

Notes