Pre-heat fan-forced oven to 220C.
Place rosemary sprigs and 1/2 of the garlic cloves on the bottom of a high-sided roasting tray.
Deeply score the fat side of the lamb, drizzle with olive oil and season with salt and pepper. Place on top of rosemary and garlic.
Cover tray with aluminium foil and place in oven. Reduce oven temperature to 150°C and cook for 4 hours or until meat pulls apart easily with a fork. (Remove foil with 30 mins cooking time to go).
With 1 hour 15 mins to go place remaining garlic cloves, sweet potato and pumpkin in a separate roasting tray and drizzle with olive oil and roast in oven.
Once lamb is cooked remove from oven and place on a chopping board. Cover with aluminium foil and a tea towel and allow to rest.
Meanwhile make gravy. Pour out oil from lamb tray and discard rosemary sprigs. Mash 2- 3 garlic cloves in the pan (discard others) 1/2 cup of the roasted pumpkin until smooth and mix into any caramelised residual flavour on bottom of pan. Add 400mLs water, onion powder, chia seeds, salt and peppers and use a fork to mix well. Place flameproof tray on stovetop (or alternatively transfer ingredients to a small saucepan) and bring to a simmer and allow gravy to reduce for 5-8 minutes, stirring regularly or until desired consistency.
Blanch or steam green beans for 5- 8 minutes or until tender. Drain.
Remove vegetables from oven and roughly mash sweet potato, pumpkin and garlic cloves with a fork.
Carve lamb and serve with vegetables and gravy.