Pound the garlic and oregano in a mortar and pestle to a coarse paste. Add 2 tsp olive oil and rub well into the lamb. Cover with cling film and marinate for 30 minutes.
Preheat the oven or lidded barbecue to 180oC. Season the lamb with salt and pepper and place in the middle of a large roasting pan.
Toss the potatoes and pumpkin with the remaining olive oil and season with salt and pepper. Scatter the vegetables around the lamb and cook for 45 minutes for medium, turning the vegetables over halfway through.
Take the lamb out and set aside to rest for 10 minutes, loosely covered with foil. Turn off the oven or barbecue and keep the vegetables in to stay warm.
Carve the lamb and squeeze some of the grilled lemon on top. Serve with the vegetables and salad.
For the grilled lemon:
Preheat a chargrill to hot and cook the lemon quarters for 11/2 minutes on each side, or until lightly charred. If you are using the barbecue you can simply cook them on the grill plate.
The most accurate way to test the temperature of the lamb is using a meat thermometer. Medium lamb should read 65oC-70oC.
For maximising flavour, try marinating the lamb overnight.
You could use a boned and butterflied shoulder of lamb instead of the leg.
If you don't want to use a chargrill pan, you can add the lemon quarters to the lamb and vegetables for the last 5 minutes of cooking time.