Soak codfish in water to cover for 4 hours to remove the excess salt. Drain well and rinse in running water.
Dry the fish. Using your fingers or forks, begin shredding the cod into a bowl.
In a saucepan, warm the oil and sautee the onions, the sliced garlic. Cook for about 5 minutes.
Add potatoes, cod, diced tomatoes, and wine.
Bring to a boil. Cook over low heat, covered, for 30 minutes, or until the potatoes are fork-tender.
How long you soak the fish is something that I cannot find a straight answer to. I have seen as little as 3 hours and up to 48 hours. I also see the need to boil the fish before cooking it, making no sense, and I skip that step.
I do not use the filet intact. This is the way that I was served this dish by my grandmother. Maybe she would shred the fish so that it will be easier to eat, or maybe it will fall apart as it cooked. Regardless of the reason, this is the way I remember eating it and this is the way I make it.
For the acid feel free to use vinegar or wine instead of lemon juice. Sometimes I use both. What I like about the lemon juice is the brightness that it brings to the dish.