Position a rack in the center of the oven and heat the oven to 375°F. Brush the beef with 1 Tbs. of the oil and 1 Tbs. of the vinegar. Season the beef all over with 2 tsp. salt, 1 tsp. of the rosemary, and 1/2 tsp. pepper. Toss the onion, fennel, and potatoes with the remaining 2 Tbs. oil, 1 Tbs. vinegar, 1 tsp. rosemary, 1/2 tsp. pepper, and 1/4 tsp. salt. Spread the vegetables over the bottom of a9x13-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°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°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.

nutrition information (per serving):

Size: based on six servings; Calories (kcal): 420; Fat (kcal): 17; Fat Calories (g): 150; Saturated Fat (g): 4.5; Protein (g): 45; Monounsaturated Fat (g): 9; Carbohydrates (g): 20; Polyunsaturated Fat (g): 1; Sodium (mg): 520; Cholesterol (mg): 95; Fiber (g): 3;