Remove the roast and ribs from the refrigerator and let stand at room temperature for 1 hour. Meanwhile, adjust an oven rack to the lowest position and heat the oven to 400 degrees.
Rub the oxtails with the tomato paste and place in a large, heavy-bottomed roasting pan. Toss the onions with 1 tablespoon of the oil, then scatter the onions in the roasting pan. Roast until the oxtails and onions are browned, about 45 minutes, flipping the oxtails halfway through the cooking time. Remove from the oven and set the roasting pan with the oxtails aside; reduce the oven temperature to 250 degrees.
When the roast has stood at room temperature for 1 hour, heat a heavy-bottomed 12-inch skillet over medium heat until hot. Meanwhile, rub the ends and fat side of the roast with the remaining 2 tablespoons oil, then sprinkle with 1½ teaspoons kosher salt (or 4 teaspoon table salt) and the pep per. Place the roast fat-side down in the hot skillet and cook until well browned, 12 to 15 minutes. Using tongs, stand the roast on end and cook until well browned, about 4 minutes. Repeat with the other end. Do not brown the side where the ribs were attached. Place the roast browned-side up on a cutting board and cool 10 minutes.
Tie the browned roast to the ribs back onto the ribs using 3-4 lengths of kitchen twine. Set the roast bone-side down in the roasting pan, pushing the oxtails and onions to the sides of the pan. Roast for 1 hour, then remove from the oven and check the internal temperature; the center of the roast should register about 70 degrees on an instant-read thermometer. (If the internal temperature is higher or lower, adjust the total cooking time.) Return the roast to the oven and cook 1.25 to 1.75 hours longer, until the center of the meat registers about 122 degrees for rare to medium-rare or about 130 degrees for medium rare to medium. Transfer the roast to a cutting board and tent loosely with foil..
While the roast rests, spoon off the fat from the roasting pan, set the roasting pan over 2 burners at high heat. Add the wine to the roasting pan: using a wooden spoon, scrape up the browned bits and boil until reduced by half, about 3 minutes. Add the beef broth, chicken broth, and thyme. Cut the twine on the roast and remove the roast from the ribs; transfer the meat to the cutting board and tent again with the foil. Add the ribs to the roasting pan and continue to cook, stirring occasionally, until the liquid is reduced by two thirds (to about 2 cups), 16 to 20 minutes. Add any accumulated juices from the meat and cook to heat through, about 1 minute longer. Discard the ribs and oxtails; strain the jus through a mesh strainer into a gravy boat, pressing on the onions to extract as much liquid as possible.
Set the meat browned-side up on the cutting board and cut into 3/8-inch-thick slices; sprinkle lightly with salt. Serve immediately, passing the jus separately.