Using sharp knife, cut through roast fat cap in 1-inch crosshatch pattern, being careful not to cut into meat. Rub 2 tablespoons salt over entire roast and into crosshatch. Transfer to large plate and refrigerate, uncovered, for at least 24 hours or up to 96 hours. Adjust oven rack to lower-middle position and preheat oven to 250°F. Season roast with pepper and arrange, fat side up, on V-rack set in large roasting pan. Roast until meat registers 115°F for rare, 120°F for medium-rare, or 125°F for medium, 3 to 3 1/2 hours. Transfer V-rack with roast to carving board, tent loosely with aluminum foil, and let rest about 1 hour. Meanwhile, increase oven temperature to 425°F. Pour off all but 2 tablespoons fat from pan. (If there isn’t enough fat in pan, add vegetable oil to equal 2 tablespoons.) Toss carrots, parsnips, Brussels sprouts, onion, thyme, 1 teaspoon salt and ½ teaspoon pepper with fat in pan. Roast vegetables, stirring halfway through roasting, until tender and browned, 45 to 50 minutes. Remove pan from oven and heat broiler. Carefully nestle V-rack with roast among vegetables in pan. Broil roast until fat cap is evenly browned, rotating pan as necessary, about 5 minutes. Transfer roast to carving board, carve meat from bones, and cut into ¾-inch-thick slices. Season vegetables with salt and pepper to taste. Serve roast with vegetables.