1. Remove roast and ribs from refrigerator and let stand at room temperature 2 hours. After an hour, adjust oven rack to lowest position and heat oven to 400 degrees F. Rub oxtails with tomato paste and place in heavy-bottomed, burner-safe roasting pan. Toss onions with 1 tablespoon oil, then scatter onions in roasting pan. Roast until oxtails and onions are browned, about 45 minutes, flipping oxtails halfway through cooking time. Remove from oven and set roasting pan with oxtails aside; reduce oven temperature to 250 degrees F.
2. When roast has stood at room temperature 2 hours, heat heavy-bottomed 12-inch skillet over medium heat until hot, about 4 minutes. Meanwhile, rub ends and fat-side of roast with remaining 2 tablespoons oil, then sprinkle with 1 1/2 teaspoons kosher salt (or 3/4 teaspoon table salt) and pepper. Place roast fat-side down in skillet and cook until well-browned, 12 to 15 minutes; using tongs, stand roast on end and cook until well-browned, about 4 minutes. Repeat with other end. Do not brown side where ribs were attached. Place roast browned-side up on cutting board and cool 10 minutes. Tie browned roast to ribs. Set roast bone-side down in roasting pan, pushing oxtails and onions to sides of pan. Roast 1 hour, then remove from oven and check internal temperature; center of roast should register about 70 degrees F on instant-read thermometer. (If internal temperature is higher or lower, adjust total cooking time.) Return roast to oven, and prepare Yorkshire pudding batter now (if making), and cook 1 1/4 to 1 3/4 hours longer, until center of meat registers about 122 degrees F for rare to medium-rare or about 130 degrees F for medium-rare to medium. Transfer roast to cutting board and tent loosely with foil. Increase oven temperature to 450 degrees F for Yorkshire pudding.
3. While roast rests, spoon off fat from roasting pan, reserving 3 tablespoons for Yorkshire puddings; set roasting pan aside while preparing puddings for baking. While puddings bake, set roasting pan over 2 burners at high heat. Add wine to roasting pan; using wooden spoon, scrape up browned bits and boil until reduced by half, about 3 minutes. Add beef broth, chicken broth, and thyme. Cut twine on roast and remove meat from ribs; re-tent meat. Add ribs, meaty side down, to roasting pan and continue to cook, stirring occasionally, until liquid is reduced by two-thirds (to about 2 cups), 16 to 20 minutes. Add any accumulated beef juices from meat and cook to heat through, about 1 minute longer. Discard ribs and oxtails; strain jus through mesh strainer into gravy boat, pressing on onions to extract as much liquid as possible.
4. Set meat browned-side up on board and cut into 3/8 -inch-thick slices; sprinkle lightly with salt. Serve immediately, passing jus separately.

Yield: 10 servings