Start with the rice. Heat the oil in the paella pan or casserole and fry the shallots and garlic, stirring all the time, for 2–3 minutes. Add the rest of the rice ingredients, except the rice itself and the fish stock. Stir, and simmer for 3 minutes. Add the stock and simmer for 2 minutes, then bring the liquid fully to the boil and add the rice. Stir everything well together and spread the rice evenly over the bottom of the pan. Lower the heat, partially cover the pan, and continue to simmer for 10–15 minutes.
While the rice is cooking, cook the seafood. Fry the onion and garlic in the oil in a large casserole or frying pan for 1 minute. Add the cod cubes and fry them, top and bottom, for 2 minutes in all. Then add the tomatoes and all the ground ingredients, with the salt and pepper and the stock or hot water. Bring the whole thing to the boil and add the mussels. Let them cook for 2 minutes, then add the rest of the seafood. Stir everything carefully together and cover the pan. Turn up the heat and leave the seafood to cook for 3 minutes only.
Uncover, adjust the seasoning of the seafood, and arrange it all on top of the cooked rice in the paella pan or casserole. Serve immediately.