Put the cous cous in a bowl and cover with tepid water. It will soften and double in size.
On a low heat, heat a medium-sized saucepan and saute the onion, garlic and chilli in oil. Pound in the mortar the salt, fennel and cumin seeds, coriander seeds and cinnamon. Add to the onion with the butter, then once melted add the finely sliced coriander stalks and the cous cous. Break it up with your spoon.
Turn the heat down to a very low heat, and add the fish, skin side up. Drizzle with olive oil, a bit of salt and pepper and throw in the lemon, cut in half. Cover with some wet greaseproof paper, and cook for 15 minutes.
To serve, sprinkle over the coriander leaves and sliced chilli; squeeze the lemon halves, dollop on some creme fraiche and drizzle with olive oil.