Step 1

Preheat oven to 425 degrees. Let steak stand until room temperature.

Step 2

Bring potatoes to a boil in generously salted water. Cook until fork-tender, about 8 minutes. Drain and let cool slightly. Brush a rimmed baking sheet with 1 tablespoon oil. Lightly crush each potato on sheet with palm to 1/2-inch thickness. Brush with 1 tablespoon oil. Season with salt and pepper, and scatter thyme on top. Roast until golden and crisp, about 25 minutes, rotating sheet once.

Step 3

Meanwhile, combine butter and scallion in a bowl. Season with salt. Heat a large cast-iron or other heavy skillet over medium-high heat, 2 minutes. Pat steak dry and rub with remaining 1 teaspoon oil. Season generously with salt and pepper and place in hot pan. Cook until browned, about 5 minutes a side. Transfer skillet to oven. Roast alongside potatoes until a thermometer inserted in thickest part of steak reads 130 degrees for medium-rare, 10 to 12 minutes. Transfer steak to a platter, top with scallion butter, and let rest 10 minutes before slicing. Serve with potatoes.