Peel the skin from the onions and cut off the top and bottoms. In a large saucepan, bring water to a boil and add the whole onions. Let it simmer on medium-low heat for 30 minutes. Drain the water and allow the onions to cool.
Brown the lamb in a pan over high heat. Lower the heat and add the tomato paste, harissa, honey and nuts. Stir over low heat for a couple of minutes, add the spices and turn off.
Combine the couscous with the pomegranate molasses and spices. Mix with the lamb mixture of keep separate for a vegetarian option.
Slice each onion halfway through the middle and peel off 5 to 6 layers large enough to fold over 2 tablespoons of stuffing. Roll the stuffed onions and place them side by side in a baking dish. Mix the tomato sauce and broth before pouring it over the onions.
Heat the oven to 350 degrees Fahrenheit and bake for 30 minutes. Sprinkle with salt, pepper and sumac. Serve with a small bowl of yoghurt on the side as a dipping option.