Rinse the vegetables in cold water. Clean the pepper, cut into strips about 2 cm long and set aside. Slice the peppers lengthways and remove the stones (if you like it spicier, you can leave them in). Then finely dice and add to the pepper strips. Halve the zucchini lengthways and cut into thin slices.
Meanwhile, cut the fish into bite-sized cubes. Peel and dice the onion as well. Put the fish fillet and the onion in the sauce and let simmer for another 5 minutes over low heat.
As soon as the fish is done, season the ragout with salt, pepper and coriander and serve garnished with coriander leaves.