Finely slice the onion, chop up the garlic. Cut the peppers into 1 cm dice. Cut in half the tomatoes. Deseed and finely chop the chilli
Heat the oil in a large, shallow casserole dish, add the onion and fry for 5 minutes until softened. Add the garlic , pepper and chilli and fry for another minute.
Add the tomatoes, and turn down the heat. Put the lid on and let it braise for a couple of minutes, or until the tomatoes release their juices.
Put the cod on the top of the tomatoes and scatter the olives around. Season with salt and pepper then grate the zest of the lemon on top and squeeze the juice over. If it's too dry add half a cup of vegetable stock or white wine.
Put the lid on, and gently braise for 5-10 minutes (depending on the thickness of the cod) until the fish turns white.
Scatter the basil leaves on top just before serving.