Thoroughly rinse the cod and pat dry. Remove any bones from the cod and cut into 6-8 pieces. Heat the olive oil in a large, wide saucepan over medium heat. Add the thinly sliced onions to the pan and cook, stirring occasionally, until soft and golden, about 10-12 minutes. 

Add the minced garlic and the black pepper to the pan, and stir until fragrant, 1-2 minutes. Add the tomato puree and the water to the pan. Stir to combine, bring to a boil, then lower the heat and simmer for 10-12 minutes.

Add the prepared cod to the pan, spooning the sauce over the cod to cover. Season to taste with more salt and pepper, and cover the pot with a lid. Cook over medium heat until the fish is cooked through and flakes easily with a fork, about 10 minutes, .

Serve hot, straight from the pan, topped with torn basil leaves. Leftover bacalao (can be in the refrigerator for 3-4 days in an airtight container.