Heat the olive oil and butter in the base of a tagine or a large, heavy-based saucepan over a medium heat, stir in the ginger, cinnamon sticks and shallots, toss to coat well and cook for 2–3 minutes until the shallots begin to colour, then add the yam and cook for a further 2–3 minutes.
Add the prunes and honey and pour in the stock. Bring to the boil, then reduce the heat, cover and cook gently for about 25 minutes. Toss in most of the herbs, season to taste, and cook for a further 5–6 minutes to let the flavours mingle. Garnish with the remaining herbs and serve immediately with plain, buttery couscous, if liked.