Peel and cut the potatoes into even-sized chunks, then place in a large pan of cold salted water. Bring to the boil, then cook over a medium heat for 15 to 20 minutes, or until the potatoes are tender.
Drain and steam dry, then mash the potatoes until really smooth (use a potato ricer if you have one). Season well and mash in the butter.
For the guacamole-style dressing, halve and destone the avocados, then scrape the flesh into a medium bowl. Season with sea salt and black pepper and squeeze over the lime juice.
Peel and grate in the onion along with the tomato, then mash together to a chunky guacamole. Chop the coriander and add it to the guacamole with plenty of extra virgin olive oil, then leave to one side.
Peel and finely chop the onion, then place into a small bowl with the lime zest and juice. Toss together and leave to one side for 5 to 10 minutes.
Finely chop the prawns and mix together with the crabmeat and aji amarillo paste, seasoning well. Mix in the onions, leaving behind the excess juice.
Line up 3 bowls – one with flour, one with beaten eggs and one with breadcrumbs.
Place a scoop of mashed potato in your hand and roll into a ball (roughly 4cm in diameter). Press a hole in the middle with your thumb, about 2cm deep, and spoon in some prawn and crab mixture.
Close the hole as gently as you can to conceal the filling, then repeat until you’ve used up all the mash and filling – you should end up with around 15 croquettes.
Roll each croquette in the flour, dip it into the egg (letting the excess drip off), then coat in breadcrumbs.
Pour the vegetable oil into a large pan and gently heat to 180°C (a cube of bread dropped into the oil will turn brown within 1 minute).
Gently lower the croquettes into the oil with a slotted spoon and fry for around 4 minutes, or until golden and crisp – you’ll need to do this in batches.
Drain the cooked croquettes on kitchen paper, then serve hot on a plate smeared with the guacamole.