Heat the oil in a pan and add the onion until soft and light gold. Add the aubergines, coat thoroughly in the oil and cook for a further two minutes. Add the courgettes and stir through, then repeat with the garlic and peppers.
Add the coriander seeds and season and cook very gently for around 40 minutes. Add the tomatoes, breaking them down gently into the pan and cook very gently for another 30 minutes until the vegetables are soft. Stir in the basil and set aside.
Meanwhile for the polenta, bring 1.5 litres (2¾ pints) water to the boil and add salt. Slowly whisk in the polenta. Cook for 5-6 minutes, taking care as it will start to spit as soon as it comes to the boil. Remove the pan from the heat and stir in the butter and Parmesan. Season and keep warm.
Preheat the oven to 200ºC/400ºF/Gas Mark 6.
Heat the oil in a frying pan, season the cod and sear on all sides in the pan. Transfer to a baking sheet and cook in the oven for 7-10 minutes until just flaking.
To serve, spoon some of the ratatouille onto a plate and top with a cod fillet. Serve the warm polenta on the side.
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