Position a rack in the center of the oven and heat the oven to 375 degrees F. Brush the beef with 1 tablespoon of the oil and 1 tablespoon of the vinegar. Season the beef all over with 2 teaspoons salt, 1 teaspoon of the rosemary, and 1/2 teaspoon pepper. Toss the onion, fennel, and potatoes with the remaining 2 tablespoons oil, 1 tablespoon vinegar, 1 teaspoon rosemary, 1/2 teaspoon pepper, and 1/4 teaspoon salt. Spread the vegetables over the bottom of a 9x13-inch Pyrex dish. Nestle the roast, fat side up, in the middle of the vegetables.

Roast the beef until an instant-read thermometer inserted in the center of the beef registers 125 degrees F for medium rare, 1 1/4 to 1 1/2 hours. Transfer the beef to a cutting board to rest for 10 min. Meanwhile, raise the oven temperature to 450 degrees F. Give the vegetables a stir and continue to roast them until they are browned and tender, about 10 min. more. Thinly slice the beef, sprinkle with salt, and serve with the vegetables.