Pour the stock and water into a very large saucepan with the onion and saffron. Bring to the boil, then cover and reduce the heat. Simmer gently for 10 minutes, then strain into a large jug.
Heat the oil in the saucepan or a paella pan and add the garlic and red capsicum. Cook gently for 2–3 minutes, then add the rice. Cook for 5 minutes, stirring often, until the rice looks transparent.
Add the wine, stir, then allow it to bubble up and evaporate. Ladle about one-third of the reserved stock into the rice. When it has all been absorbed, add a further third, then cook gently until it has been absorbed.
Add the remaining ingredients (except the lemon wedges and extra chopped parsley) to the pan, along with the rest of the stock. Cook gently, stirring occasionally, for about 10 minutes until all the seafood is cooked and the rice and vegetables are tender, adding extra stock or hot water, if needed.
Season to taste and tip into a serving dish if not cooked in a paella pan. Garnish with lemon wedges and extra chopped parsley.