Combine salt, pepper and garlic and rub the mixture into the surface of the beef.
Place the beef in a roasting pan, fat side up, and roast for 1 hour.
Reduce the oven temperature to 400 degrees and continue roasting for about 1 1/2 hours, or until a meat thermometer inserted in the center of the meat registers 130 degrees for medium-rare or 140 degree for medium.
About 30 minutes before the roast will be done, begin preparing the mushrooms.
Soak the dried mushrooms in the warm water until they are completely soft and completely rehydrated, about 15 minutes.
Squeeze out the mushrooms, rinse well, and set aside.
Strain the soaking liquid through a coffee filter.
Combine the strained soaking liquid and the beef broth in a saucepan, place over high heat and reduce to 1 cup; set aside.
Saute the leeks in butter in a large deep skillet until softened.