To make the stock, heat the oil in a large saucepan. Add the leeks, carrots, onion, celery, garlic and bay leaves. Stir in the tomato purée, and the choricero and ñora peppers. Cook over a low-medium heat for 45 minutes, stirring frequently, until the vegetables are thoroughly caramelized. Add the prawns and the paprika, stir, then add the wine, Ricard, brandy and sherry. Turn the heat up to evaporate the alcohol, then add just enough water to cover the vegetables and simmer gently for a further 45 minutes. Remove from the heat.
Strain the stock through a sieve over a large bowl, squeezing down on the prawns and vegetables.
To make the picada, put the garlic and parsley into a pestle and mortar and bash together, then add the nuts and keep bashing until everything is in little pieces. Add a little olive oil to loosen if necessary.
To make the stew, heat 50ml of olive oil in a large pan on a medium heat, then add the onions and cook gently for 5 minutes, until soft but not coloured. Add the saffron and stir for a minute. Add the tomatoes and cook for a further 5 minutes, then add the potatoes. Add the stock and simmer for 7 minutes, until the potatoes are cooked, then turn the heat down to medium-low.
Put the remaining oil into a flat pan on a medium-high heat. Season the squid and fry quickly so that it colours slightly, then add to the pan with the stock. Do the same with the crab pieces, the langoustines and finally the fish. Turn the heat down to low, then add the mussels and clams to the stew and half cover.
When the stew is very gently bubbling, stir in the picada and leave for a couple of minutes. Taste and adjust the seasoning if necessary, then turn off the heat.
Toast the bread on both sides in a pan with a little olive oil until nice and crispy, then rub with the raw garlic clove.
Serve in bowls, with the toast for dipping.