Put 5 litres (175 fl oz/20 cups) of water, the celery, leek, garlic and onion in a large stockpot and bring to the boil. Plunge the octopus into the boiling water three times for 20 seconds each time. After the third repetition, wait until the liquid returns to the boil, add the octopus and cook for approximately 1 hour. Remove octopus and transfer to an ice bath. Allow to cool, cut off the legs and refrigerate.
To make the coconut milk sauce, put the coconut flesh and 1.5 litres (52 fl oz/6 cups) of water in a large saucepan. Bring to the boil, then remove from the heat. Blend in a food processor and strain through a fine sieve, pushing out as much coconut milk as possible. Set aside.
Put the onion, capsicum, garlic, aji dulce, annatto seed paste, tomato paste and coriander in a frying pan with the butter. Cook over low heat for about 10 minutes until soft. Add the coconut milk and reduce until you have about 200 ml (7 fl oz) and the liquid is the consistency of a sauce. Strain into a bowl, adding half of the solids. Season generously with salt, pepper and lemon juice. Allow to cool completely before adding the octopus legs. Cover and refrigerate until ready to serve.
To make the spiced mustard, combine the English mustard and smoked vegetable hot sauce in a bowl and stir until well combined. Set aside until ready to serve.
To prepare the red onion ceviche, peel the onion and slice it 3 mm (⅛ inch) thick on a mandolin. Put all the ingredients into a bowl, stir gently to combine, cover and set aside until ready to serve.
To make the plantain chips, slice the plantain 2 mm (1/16 inch) thick on a mandolin. Put the butter in a frying pan over low heat. Add the garlic and aji dulce and sweat until soft, with no colour. Remove from the heat, add the tomato concasse and parsley. Season with salt and pepper and reserve. Heat vegetable oil in a deep-fryer or large, heavy-based saucepan and heat to 180°C (350°F) or until a cube of bread dropped into the oil turns brown in 15 seconds. Add the plantain slices and cook for 3 minutes. Remove, drain on paper towel and then combine with the tomato mixture.
Put the marinated octopus legs in a sous-vide bag and cook in a water bath for 10 minutes at 85°C (185°F). Gently remove from the bag: the octopus should look glazed. To serve, place the octopus in the centre of the plate. Stack about 8 plantain chips next to the octopus. Serve with several drops of spiced mustard and a generous spoonful of the red onion ceviche.