Take a small sharp knife and pierce the lamb leg across the surface about an inch (2cm) deep.
In each hole, slide in a slice of fresh garlic and a sprig of rosemary. Drizzle some extra virgin olive oil over the lamb leg, and pat it with your hands to make sure there is an even coverage. Season it with salt and pepper.
Dice the onion, carrot, potatoes, and pumpkin into equally sized pieces about 1-inch square (2cm).
Add the carrots, potatoes, and pumpkin to a bowl then drizzle with a small amount of olive oil, sprinkle with salt and pepper, to taste, then toss gently to cover all the vegetables evenly.
Preheat the Dutch oven to about 350 degrees F (180 degrees C). Drizzle a small about of olive oil on the base. Sprinkle the base with the vegetables, place the leg of lamb on top and nestle in the vegetables. Place the lid on the Dutch oven.
Place the Dutch oven in a barbecue pre-heated to 350 degrees F (180 degrees C) then leave about 25 minutes per 1 pound (1/2 kg) of meat. Check doneness with a meat thermometer. Rare is 140 degrees F (60 degrees C), medium 150 to 160 degrees F (65 to 70 degrees C), well-done 165 degrees F (75 degrees C).