• Remove roast from refrigerator and let stand at room temperature 2 hours. Adjust oven rack to lowest position and heat oven to 425 degrees. 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.
• Rub roast with remaining 2 tablespoons oil, then season with 1 1/2 teaspoons kosher salt and fresh coarse ground black pepper. Set roast bone-side down in roasting pan pushing oxtails and onions to sides of pan. Roast 1 hour, then check internal temperature; center of roast should register about 70 degrees on instant-read thermometer. (If internal temperature is higher or lower, adjust total cooking time.) Lower oven temperature to 300 degrees, return roast to oven, cook 1 1/4 to 1 3/4 hours longer, until center of meat registers about 120 degrees for rare to medium-rare or about 130 degrees for medium-rare to medium. Transfer roast to cutting board, tent loosely with foil and let rest while you prepare the jus.
• While roast rests, spoon off fat from roasting pan and set roasting pan over 2 burners over high heat. Add wine to roasting pan; using wooden spoon, scrape up browned bits and simmer 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), Remove ribs and oxtails and set aside; strain jus through mesh strainer into a saucepan, pressing on onions to extract as much liquid as possible. Set aside until ready to serve.
• Set meat browned-side up on board and cut into 3/8 -inch-thick slices; sprinkle lightly with salt. Serve immediately, passing the jus separately.

Optional Mushroom Jus (Our Families Favorite)

• Heat a large skillet over medium high heat.  Add beef fat drippings and sliced mushrooms.  Allow the mushrooms to cook undisturbed until golden brown, 1 – 2 minutes (depending on the heat of your pan).  Stir, and continue to cook over medium high heat for an additional 2 minutes.  Add butter, scallions and cook an additional 2 – 3 minutes.  Remove skillet from heat and stir in ¼ cup parsley.  Stir into jus mixture.
• Add to the jus mixture.  Finish with the remaining parsley.