Start by cutting the potatoes into even-sized chunks, then placing them in a large pan of cold salted water. Bring to the boil, then cook over a medium heat for 15–20 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 a knob of butter.
Now make the guacamole-style dressing. Scrape the avocado flesh into a medium-sized bowl, discarding the stones and skins, season with salt and pepper and squeeze over the juice of the 2 limes. Grate in the onion and 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.
Place the finely chopped red onion in a small bowl, add the juice and zest of the 1 lime, toss together and leave to one side for 5–10 minutes. Finely chop the prawns and mix together with the crabmeat and aji amarillo paste, seasoning well. Mix the onions into the bowl with the prawns, leaving behind the excess juice.
Line up 3 bowls – one with the flour, one with the beaten eggs and one with the 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. Next, 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.