How to Make It
In a small bowl, combine the rosemary with the 3 tablespoons of black pepper and the vegetable oil and rub all over the roast. Cover and refrigerate overnight.
Put a large roasting pan in the middle of the oven and preheat to 375°. Let the rib roast stand at room temperature for 30 minutes.
Season the roast with salt and put it in the hot roasting pan, fat side down. Roast for 30 minutes. Turn the roast over and cook at 350° for about 3 hours, rotating the pan 2 or 3 times. The roast is done when an instant-read thermometer inserted in the thickest part of the meat registers 130° for medium-rare. Transfer the roast to a carving board to rest for at least 20 minutes.
Meanwhile, in a saucepan, melt the butter. Add the shallot and garlic and cook over moderate heat until lightly browned, 5 minutes. Add the wine and boil for 2 minutes. Add the stock, vinegar and thyme and bring to a boil. Simmer over moderate heat until reduced to 2 1/2 cups, about 20 minutes. Strain the sauce and return it to the saucepan. Whisk in the porcini powder and simmer for 1 minute. Cover, remove from the heat and let stand for 5 minutes. Season with salt and pepper. Carve the roast and serve with the porcini jus.
The porcini jus can be refrigerated overnight. Reheat gently.