Rinse with cold water and pat the boneless leg of lamb dry with paper towels.
Combine the ingredients for the marinade and coat the entire leg of lamb, marinate the meat for an hour or overnight.
Get the charcoal grill started. You want to cook with indirect heat, which means we're putting the charcoal briquets off to one side of the grill or using a horseshoe shape.
The grill temperature should be maintained between 375 F and 400 F.
Place the sliced onions on the bottom of a cast-iron Dutch oven, and place the marinated leg of lamb on top.
Brush on additional marinade once every hour.
Position the Dutch oven away from the hot coals, close the lid, and cook for 2 hours or until an instant-read thermometer reads 120°F for rare, 125°F medium-rare, 135 °F medium, 145°F for medium-well, and 155 °F for well-done.
Remove the Dutch oven from the grill and allow the leg of the lamb to rest for fifteen (15) minutes.
Serve.