Mix the fish with all the other ingredients up to and including the pepper, knead well by hand until the ingredients have combined to form a compact mass. Shape the mixture into 4 burgers, flatten a little. Grill the burgers over/on a high heat (approx. 220 °C for approx. 5 mins. on each side.
Combine the yoghurt with all the other ingredients up to and including the pepper, refrigerate.
Grill the pitas over/on a medium heat (approx. 200 °C for approx. 2 mins. on each side. Open the pitas, fill with the dressing, burgers, lettuce, avocado, onion and onion sprouts.