Chop the fish with a cleaver into small pieces. Place the fish in a bowl and cover with cold water. Soak for three days, changing the water 1-2 times a day.
Drain the fish and place it in the milk for 3 hours. Pat dry. Melt the butter with the oil in a pan and fry the fish. In the meantime, peel the onion and cut into strips. Peel and finely chop the garlic cloves. Cut the tomatoes into cubes. Add the tomatoes with some bay leaf to fish and let everything simmer about 15-20 minutes at a low temperature. Season with pepper to taste and serve as desired with polenta.