Finely dice the cucumber, red onion, tomatoes, and jalapeño, then finely slice the coriander.
Peel the avocado and cut it into rough chunks.
Cut the prawns into rough chunks, then place them in a bowl. Cover with the juice of the 3 limes. Add the sherry vinegar if desired. Refrigerate for 15–20 minutes (you will see the prawns start to cook in the lime juice and change colour).
Add the rest of the vegetables to the prawns, along with the olive oil, sugar, salt, and pepper. Finally, mix in the avocado.
Leave this in the fridge for at least half an hour before serving with tortilla chips.
Note: If you would like your prawns to be more cooked/firmer, leave the ceviche in the fridge for 2 hours before serving!