Peel and finely chop the onion. Clean and wash the fennel and cut into thin slices. Wash, clean and halve the tomatoes. Wash fish, dab dry and sprinkle with lemon juice.
Season with salt and pepper. Heat the oil in a pan and fry the fish in it at medium heat for 10-12 minutes. Add the onions and fennel after about 5 minutes and fry. Wash the rosemary, dab dry and pluck the leaves from the twig.
Just before the end of the frying time, add the tomatoes and olives and mix in. Season with salt and pepper. Sprinkle fish with coloured pepper and serve garnished with rosemary.