Preheat the oven to 375 degrees F (190 degrees C).
Rub the leg of lamb all over with salt and pepper. Use a small knife to make punctures in the lamb about 1 inch apart. Press slivers of garlic into each hole so that they are about 1/2 inch below the surface.
Place the meat in a roasting pan. You can either remove the rosemary from the stalk and sprinkle and rub into the meat on all sides, or you can simply use the string from the lamb to secure against it.
Pour the can of tomato sauce over the whole thing.
Bake for 45 minutes in the preheated oven, then lower the temperature to 325 degrees F (160 degrees C), and continue roasting until the internal temperature of the meat is at least 160 degrees F (70 degrees C), about 15 minutes. If you want the meat well done, wait until the internal temperature reaches 170 degrees F (75 degrees C).
