Put the red mullet and gurnard in a large bowl. Add the onion, fennel, garlic, leek, extra virgin olive oil, herbs, salt, spices and plum tomatoes and mix well. Cover and marinate in the fridge for 24 hours, turning the contents every 4–6 hours.
Separate the fish from the vegetables and spice mix, reserving the liquid. Heat 2 tablespoons olive oil in a large frying pan over a medium heat and fry the fish, discarding the heads and tails, for 10–15 minutes, or until golden brown on all sides.
Meanwhile, heat the remaining 1 tablespoon oil in a saucepan large enough to hold the soup. Add the vegetables and sweat for 5–7 minutes, without allowing them to colour. Season with salt and pepper.
Remove the fish from its pan and keep warm. Deglaze the pan with a splash of the wine and add to the vegetables.
When the vegetables are soft, add the tomato purée and cook for 5 minutes. Add the chopped tomatoes, reserved liquid and remaining wine, increase the heat and simmer for 10 minutes. Add the stock, bring to the boil rapidly, reduce to a simmer and cook for 20 minutes.
Meanwhile, cook the potatoes in a large pan of salted water for 20 minutes, or until soft.
Blend the soup in a blender until very smooth. Pass through a fine sieve and keep warm.
Blanch the whole leeks in a large pan of salted water for 5 minutes. Drain and slice into 1cm/½in slices.
Drain the potatoes and add to the soup along with the leeks.
Bring a large saucepan of salted water to the boil, then add the langoustines and cook for 1 minute. Drain and set aside to cool slightly before removing the tail meat, then add to the soup to cook through.
Sear the monkfish tail in a hot frying pan for 5 minutes, until golden on each side but not quite cooked through. Remove from the pan and cut into 24 slices. Add to the soup to cook through.
Season the scallops in the same pan, sear and cook them through.
To serve, divide the soup between 12 warmed serving bowls and top each one with a seared scallop. Drizzle the fish with lemon juice and serve alongside.