Heat the smen or ghee in the base of a tagine or a large, heavy-based saucepan over a medium heat, stir in the onions and garlic and cook for 2–3 minutes to soften them. Add the lamb and stir to coat well, then add the turmeric, cinnamon and ras el hanout.
Pour in enough water to almost cover the meat and bring it to the boil. Reduce the heat, cover and cook gently for 1 hour. Add the dates and stir in the honey, season with sea salt and black pepper then re-cover and cook gently for a further 20 minutes.
Heat the oil with the butter in a small, heavy-based frying pan over a medium heat, toss in the pistachios and cook for 1–2 minutes until they are lightly browned.
Scatter the pistachios over the lamb, garnish with the flat leaf parsley and serve immediately.