Fill a large pot with 1.5 litres of cold water and add the fish and spices. Quarter one of the onions and add it to the pot with two whole garlic cloves, the quartered lemon, chopped celery and sprigs of parsley and coriander. Add a pinch of salt and bring to the boil. Reduce the heat to low and simmer for 20 minutes. Remove the fish from the pot and set aside, then strain the broth into a bowl.

Chop the remaining onion and garlic and add them to a clean saucepan with the olive oil. Season with salt and pepper. Cook over a low to medium heat for about 10 minutes, stirring often, until the onion has softened. Add the diced potatoes and tomatoes and cook for a further 3 to 4 minutes, until the tomatoes have softened. Pour half the broth into the pan and bring to the boil. Reduce the heat slightly and simmer until the potatoes are tender and cooked through.

Using a handheld blender, blitz the vegetables and broth together into a smooth soup, adding more of the reserved cooking broth as needed. Carefully remove the skin and bones from the fish and flake it into chunks. Add the fish to the soup with a squeeze of lemon and a pinch of cayenne. Serve in large bowls, garnished with the remaining parsley and coriander. Drizzle with extra virgin olive oil before serving.