In a small bowl mix together the garlic, cumin, pomegranate molasses, olive oil, lemon, honey and thyme and season. Lay the leg of lamb flat and pour the marinade over, cover and place in the fridge for at least 30 minutes.
Heat the oven to 220°C.
Slice the onions into 2cm wedges and scatter over a medium roasting tray, lay the lamb on top. Place in the oven for 20-30 minutes, depending who pink you like it, take out and leave to rest for 15 minutes.
Thinly slice and lay on a serving platter and drizzle over the juice, scatter with coriander leaves and pomegranate seeds if using to serve.