Heat the oil in the base of a tagine or a large, heavy-based saucepan over a medium heat, stir in the onion, ginger and cumin seeds and cook for 2–3 minutes until the onion starts to soften. Add the sweet potatoes and stir to coat well.
Pour in the lemon juice, then cover and cook gently for 15 minutes. Add the olives and the orange blossom water, re-cover and cook for a further 10–15 minutes to let the flavours mingle. Season to taste with salt and pepper, then stir in the coriander. Serve with couscous, if liked, or as an accompaniment to a meat or poultry tagine.