Heat olive oil to the verge of smoking in a skillet.
Dip pieces of cod in flour and fry lightly on both sides.
Remove to a plate.
In the same oil, fry the onion slowly until it is yellow and soft.
Fry tomatoes, peeled and chopped, over a moderate flame until the sauce has thickened.
Add two cloves of pressed or crushed garlic, cook 2 minutes longer, reheat cod in sauce, and serve.