Rinse with cold water and pat the boneless leg of lamb dry with paper towels.
In a bowl, whisk together olive oil, honey, mustard, minced garlic, ramps (or substitute), black pepper, and salt. Reserve 1/4 cup of the marinade for basting, then pour the remaining marinade over the lamb and marinate for at least 1 hour or overnight.
Get the charcoal grill started. If you want to cook with indirect heat, put the charcoal briquets off to one side of the grill or use 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.
Position the Dutch oven away from the hot coals, close the lid, and cook for 2-3 hours, or until an instant-read thermometer reads 120°F for rare, 125°F for medium-rare, 135°F for medium, 145°F for medium-well, and 155°F for well-done.
Brush on additional marinade once every hour (As needed).
Remove the Dutch oven from the grill and let the lamb's leg rest for fifteen (15) minutes.
Serve.