For the lamb
Place the lamb in a large freezer bag along with the molasses, garlic, cumin, coriander and chilli and leave overnight in the fridge. Remember to turn the meat from time to time while it marinates. Preheat the oven to 230°C/450°F/Gas mark 8.
Put the peeled and halved onions onto a large roasting tin and place the lamb on top, along with all the marinade. Place in the hot oven for 20 minutes and then reduce the heat to 150°C/300°F/Gas mark 2 and add the wine and water. Cover with tin foil and cook for another 4 hours. Let your meat rest for 20 minutes before serving. The meat should be tender enough to serve with a spoon or a fork, also include the onions.
For the gratin dauphinois
Before you prepare the dish, liberally butter a gratin dish and preheat the oven to 180°C/356°F/Gas mark 4.
Peel and thinly slice the potatoes, which is best done in a food processor. Clean the leeks and finely shred them. Start with a layer of potatoes, overlapping rows on the bottom of the dish. Season well with salt and pepper, before scattering on some of the leeks and gruyere cheese. Continue this process, building up layers and ending with a layer of potatoes and cheese.
Mix the cream with the milk and pour over the potatoes. Place in the middle of the oven and bake for 60-90 minutes or until the potatoes are tender. If it starts to brown too much, cover with foil.