Squeeze the lemon over a bowl and set the bowl aside. Cut the lemon in two, remove the pith from one half and chop the rind.
Heat the oil in a non-stick frying pan and fry the onion, fennel, ginger and garlic until soft. Add the chicken and fry for a few mins more until just browned.
Add the lemon rind and spices, then pour in the stock. Cover and simmer for 25 mins, then add the tomatoes, olives, coriander and half the lemon juice. Cook for 5 mins more until the fennel and chicken are tender and the sauce reduced.
Meanwhile, cook the couscous following pack instructions and stir in the chickpeas. Add more lemon juice to taste, then serve the tagine with the couscous, extra coriander and fennel fronds.