Heat the smen or ghee in the base of a tagine or a large, heavy-based saucepan over a medium heat, stir in the ginger and onion and cook for 2–3 minutes to let the flavours mingle. Add the beef, stir to coat well and cook for a further 1–2 minutes, then stir in the ras el hanout.
Pour in the stock and bring to the boil, then reduce the heat, cover and cook gently for about 40 minutes to cook the meat and flavour the stock.
Add the sweet potatoes and top up the liquid with water, if necessary, to just cover the beef and sweet potatoes. Re-cover and cook gently for a further 25–30 minutes until they are tender. Season to taste and garnish with the coriander. Serve immediately with plain, buttery couscous, if liked.