Heat the oil in the base of a tagine or a large, heavy-based saucepan over a medium heat, stir in the onion, chilli, garlic and cumin seeds and cook for 2–3 minutes to let the flavours mingle. Stir in the leeks and cook for 2 minutes, then add the thyme, sage and plum tomatoes.
Pour in the stock and bring to the boil, then reduce the heat, cover and cook gently for 15 minutes. Season with salt and pepper, then add the fish chunks, re-cover and cook gently for a further 5–6 minutes until the fish is cooked through. Serve with couscous, if liked.