Soak the salt cod in a bowl of cold water for 1–2 days, changing the water several times. Drain the cod and cut into small chunks, discarding any skin and bones.
Heat 2 tablespoons of the oil in a large saucepan and fry the onions and aubergine very gently, stirring frequently, for about 10 minutes until lightly browned, adding the garlic for the last couple of minutes.
Add the stock to the pan and bring to a gentle simmer. Stir in the oregano, tomato purée and salt cod, cover and cook very gently for about 25 minutes until the cod is very tender.
Stir the spaghetti into the pan and cook for 5 minutes until almost tender. Stir in the tomatoes and cabbage leaves and cook for a further 5 minutes. Season to taste with salt and pepper and serve in bowls.