Preheat the oven to 180°C, gas mark 4.
In a small bowl, whisk together the olive oil, sumac, smoked paprika, lemon juice, crushed garlic, brown sugar, rosemary leaves and balsamic vinegar. Season with salt and pepper.
Using a small, sharp knife, score the leg of lamb slightly.
Place the lamb in a rimmed baking tray along with the halved garlic heads and the peeled, quartered onions. Drizzle the garlic and onion with olive oil and season with salt and pepper.
Brush the leg of lamb generously with the oil mixture.
Roast the lamb for about an hour, brushing with the oil mixture every 15 minutes.
Test the internal temperature of the thickest part of the meat with a meat thermometer after an hour, the meat should be 55-60°C for medium.
Let the lamb rest for about 20 minutes before serving.
Serve with beetroot and sweet potato spirals; roasted butternut and green bean salad; quinoa, feta and pomegranate salad; or roast paprika, chickpea and plum salad.