Step 1

Adjust oven rack to lowest position and heat oven to 425 degrees. Toss potatoes, 2 tablespoons oil, ½ teaspoon salt, and ½ teaspoon pepper together on rimmed baking sheet. Arrange potatoes cut side down and roast for 10 minutes. Toss broccolini with 2 tablespoons oil and ½ teaspoon salt and scatter over potatoes. Roast until vegetables are tender, 15 to 20 minutes.

Step 2

Meanwhile, heat 12-inch cast-iron skillet over medium-high heat for 5 minutes. Cook scallions until charred on both sides, about 5 minutes per side. Transfer scallions to cutting board. Pat steaks dry with paper towels and sprinkle with ¾ teaspoon salt and 1 teaspoon pepper. Add 1 tablespoon oil to skillet and swirl to coat. Add steaks and cook, flipping every 2 minutes, until well browned and registering 120 to 125 degrees (for medium-rare), 10 to 13 minutes. Transfer steaks to cutting board and tent with foil.

Step 3

Chop scallions fine, then whisk together with horseradish, vinegar, and remaining ¼ cup oil, ¼ teaspoon salt, and ¼ teaspoon pepper. Slice steaks and serve with scallion sauce and vegetables.