Wash the courgettes, cut slightly lengthways and season with salt. Clean the fish and season with salt. Fry the courgettes for a while, then add cooking oil on a pan and place the fish in the middle and courgettes all around. Cook the rest of the cooking oil with the onions and the tomatoes (grounded) and pour this sauce on top of the fish and the courgettes. Add a cup of water and some pepper and cook in a moderate oven temperature.