Preheat an oven to 450 degrees F (230 degrees C).

Slice small cuts along the outside edges of the roast; fill cuts with garlic slices. Generously salt the entire roast. Spread a thin layer of Dijon mustard over the entire roast; generously sprinkle mustard layer with rosemary leaves. Place roast, bone side-down, in a baking dish.

Bake in the preheated oven for 30 minutes. Reduce oven temperature to 350 degrees F (175 degrees C). Place a meat thermometer in the thickest part of the meat not touching the bone. Cook to your desired degree of doneness, or an internal temperature of at least 145 degrees F (65 degrees C) for medium, about 1 1/2 hours. Remove from the oven, cover with a doubled sheet of aluminum foil, and allow to rest in a warm area 10 to 15 minutes before slicing.