Combine the salt, pepper, thyme, granulated garlic and mushroom powder in a small bowl. Mix well.
Season the beef on all sides with the salt mixture. Wrap the beef in butcher paper and place in the refrigerator 12 hours, or up to 36 hours.
Preheat the oven to 250 degrees F.
Remove the roast from the refrigerator and pat dry with paper towels. Season the beef with the remaining salt mixture.
Scatter the onions, rosemary and garlic in the center of the roasting pan and drizzle with oil. Put the meat bone-side down on top of the vegetables and place in the oven. Cook until the meat reaches an internal temperature of 110 to 115 degrees F, 3 to 3 1/2 hours (use an instant-read thermometer!!). Remove the meat to a rack and loosely cover with aluminum foil.
Turn the oven to 500 degrees F. While the oven heats, place the roasting pan on the stovetop over medium heat. Drain any excess fat from the pan, if necessary. Add the beef stock to the pan and bring to a boil, scraping up any browned bits. Reduce the heat to a simmer and cook for 5 minutes. Strain into a smaller saucepan and bring to a simmer.
Combine the cornstarch and water and mix until smooth. Whisk the cornstarch mixture into the beef stock and return to a simmer. Whisk the butter into the sauce.
Return the beef to the oven on a rack set over a roasting pan. Roast until well browned on all sides, about 10 to 12 minutes. Remove the meat from the oven and rest 10 minutes before slicing. To slice, remove the strings. Remove the meat from the rack of bones. Put the bones on the rack (with the rawer side up) in the roasting pan and cook until crispy on top, 4 to 5 minutes (watch carefully). Slice the meat into 1/2-inch slices. Pour the gravy into a serving vessel. Serve with the bones, if desired.