Sprinkle the roast with salt and pepper. Coat with the Dijon mustard. Set aside about 1 tablespoon of the ground mushroom powder for the sauce. Press the remaining powder on the meat to make a crust.
In a heavy oven-proof pan, lay the roast, bone-side down, on a bed of rosemary, thyme, shallots and garlic. Drizzle with the olive oil. Add the veal stock to the pan. Put the pan in the oven and cook the beef for 15 minutes.
Reduce the heat to 325 degrees F and cook for a total of 2 hours for medium-rare, or approximately 20 minutes per pound. Check the internal temperature of the roast in several places with an instant-read thermometer; it should register 125 degrees F for medium-rare. Remove the beef and place on a carving board. Cover with aluminum foil and let rest for 20 minutes. The internal temperature of the meat will continue to rise by about 10 degrees. Remove the vegetables and keep warm.
For the sauce:
Heat the oil on medium heat and saute the shallots for about 2 minutes. Add the mushrooms and cook until slightly brown, about 5 minutes. Strain the juices from the rib roast pan into the shallot-mushroom mixture, and then add the veal stock and let thicken for another 5 minutes. Whisk the butter into the sauce. Add the reserved 1 tablespoon mushroom powder. Season with salt and pepper.