1/4 cup extra-virgin olive oil
1/2 cup finely chopped fresh rosemary leaves
6 garlic cloves, minced
1 teaspoon finely grated lemon zest
Coarse salt and ground pepper
2 pounds small potatoes, halved or quartered if large
Preheat oven to 300 degrees.
In a small bowl, stir together 3 tablespoons oil, rosemary, garlic, and lemon zest; season with salt and pepper.
In a large ovenproof skillet, heat 1 tablespoon oil over medium-high. Pat beef dry and add to skillet. Increase heat to high and cook, turning with tongs, until beef is browned on all sides, about 10 minutes.
Brush or spoon rosemary mixture over roast and place in oven. Roast 20 minutes.
Remove from oven and place potatoes around roast, tossing in pan juices. Roast until potatoes are tender and beef is medium (an instant-read thermometer inserted in thickest part of roast should read 120 degrees to 125 degrees), 30 to 35 minutes.
Transfer roast to a cutting board, loosely tent with foil, and let rest 10 minutes. Toss potatoes with pan juices; tent with foil. Thinly slice meat against the grain and serve with potatoes and horseradish sauce.