Cut the cod into 5 cm chunks. Pat them dry with some kitchen paper and flour them on both sides.
Peel the potatoes and cut them into thin slices, then flour them too. Fry the cod chunks in a large non-stick pan until slightly golden. At the end, fry the potatoes too.
Set aside in a dish with some cooking paper to soak up the extra olive oil.
In the meantime, make the tomato sauce. Heat 3 tablespoons of extra virgin olive oil in a non-stick pan with a whole garlic clove and some finely chopped onion. Add 2 tablespoons of water to cook the onion through: it is supposed to be tender. Add the tomatoes, cut into small pieces, season with pepper and cook for at least 15 minutes, until it becomes a liquid but thick tomato sauce.
Put cod and potatoes in a greased saucepan and cover with tomato sauce. Add salt only if needed, usually the cod is already salted.
Reduce to a simmer and cook for about 5 minutes, so that all the flavours will be mixed together. Sprinkle with some parsley and serve hot.