Peel and finely chop the onion. Peel and dice the sweet potatoes into 1cm-thick chunks. Put a medium pan on a medium heat for 1 min, then add 1 tbsp olive oil, the chopped veg and a pinch of salt and pepper. Gently fry, stirring now and then, for 5 mins till the veg start to brown and look glossy.
While the veg fry, peel and crush the garlic. Roughly chop the dates. Finely grate the zest from the lemon. Finely chop the coriander stalks (keep the leaves for later). Drain and rinse the chickpeas.
Stir the garlic, dates, lemon zest and coriander stalks into the pan. Add 2 tsp ras al hanut. Cook and stir for 1 min till the pan smells fragrant.
Stir in the chickpeas. Crumble in the stock cube. Squeeze in the juice from half the lemon. Pour in 300ml boiling water. Pop on a lid, bring to the boil and then turn the heat down a little. Let the tagine simmer for 15 mins, stirring now and then, till the sweet potatoes are tender and have started to break down.
While the tagine simmers, tip the couscous into a heatproof bowl. Pour over 300ml boiling water. Cover the bowl with a plate and set aside to soak. The couscous will suck up all the water, which will ‘cook’ it.
Taste the tagine and add a squeeze more lemon juice, or a pinch more salt or pepper if you think it needs it. Fluff the couscous up with a fork and divide it between 2 warm plates. Top with the tagine and garnish with coriander leaves.