Using a sharp knife, make 1/2-inch-deep incisions in the lamb every 2 to 3 inches on both sides of the leg; press half a garlic clove into each hole. Season generously with salt and pepper and let stand at room temperature for 2 hours.
Preheat the oven to 500 degrees . Arrange the lamb stew meat and onion in a heavy-duty roasting pan and place the lamb leg flat side down on the mixture. Roast, turning once, until browned, 35 to 40 minutes. Lower the oven temperature to 350 degrees , turn flat side down again and cook, checking the internal temperature on an instant-read thermometer every 15 minutes until it registers 125 degrees for medium-rare, about 50 minutes (the temperature will continue to rise). Transfer to a carving board with a well, tent loosely with foil and let rest for 15 to 20 minutes.
Meanwhile, stir the marjoram into the lamb-onion mixture and roast until the meat is well-browned, about 10 minutes.
Using a turkey baster, transfer the pan juices to a measuring cup and let the fat rise; skim and discard. Return the juices to the pan. Stir in the beef broth, place the pan over high heat and cook until the liquid has almost completely evaporated. Add 1 cup water and cook, scraping up all the browned bits. Strain into a saucepan, pressing to extract the juices from the solids; discard the solids. Rewarm the sauce.
Carve the roast, adding any carving juices to the sauce.