Desalt the capers: rinse them repeatedly under running water.
Then check to see that there are no bones in the salt cod fillet. Cut the piece of cod into 4 pieces of about 3 cm each keeping the skin on as it keeps the fish together during cooking.
Then, peel and finely slice the onion. Place a saucepan on the stove, add the oil and then the onions and soften them over a low heat, stirring occasionally for about 4-5 minutes. As soon as they are softened, set aside the onions being careful not to leave the oil in the pan and flour the pieces of cod quickly, shaking off any excess flour. Increase the heat and then add the pieces of cod to brown, for about 1-2 minutes per side, then turn them as soon as they are golden.
Add the white wine, and once it evaporates, lower the heat and add the tomato passata.
Add the softened onions to the sauce and then add the olives and desalted capers, season with the oregano and a pinch of salt and pepper.
Mix very delicately, cover and simmer for about 40 minutes over a low heat. Add parsley to taste.