How to Make It
Preheat oven to 450°F. Toss sweet potatoes, 2 tablespoons oil, ½ teaspoon salt, and ¼ teaspoon pepper on a rimmed baking sheet; spread in a single layer. Bake until browned and tender, about 30 minutes.
Meanwhile, combine butter, cheese, garlic, and ½ teaspoon pepper in a small bowl. Set aside.
Heat remaining 1 tablespoon oil in a large skillet over medium-high. Season steak with remaining ¾ teaspoon salt and ½ teaspoon pepper. Cook, flipping once, until well browned and a thermometer inserted in thickest portion registers 135°F, 4 to 5 minutes per side. Transfer to a cutting board and let rest for 5 minutes.
Add spinach to skillet and cook, tossing constantly, until wilted, about 1 minute. Thinly slice steak against the grain and top with Parmesan butter. Serve with sweet potato fries and spinach.