Pat the desalted cod fillets dry with kitchen paper and set aside.
Heat a splash of extra virgin olive oil in a large frying pan or shallow casserole over medium heat. Add the chopped onion, 2 sliced garlic cloves and chopped green pepper. Cook gently over low–medium heat for 10–15 minutes, stirring occasionally, until the vegetables are soft and translucent.
If using, pour in 50 ml dry white wine. Increase the heat slightly and cook for about 2 minutes, until the alcohol has evaporated.
Stir in the crushed tomatoes (or passata) and 1 teaspoon caster sugar. Season to taste with salt and freshly ground black pepper (remember the cod will already be salted). Reduce the heat to low and simmer gently for 15–20 minutes, stirring occasionally, until the sauce has thickened.
Season the cod fillets lightly with black pepper. Spread the sauce evenly in the pan and place the fillets, skin-side up, on top. Cover the pan and cook gently for 8–10 minutes, or until the fish flakes easily.
Remove from the heat, sprinkle with chopped fresh parsley and serve with boiled potatoes, steamed rice or crusty bread.