Pull the meat from the chicken and shred it roughly. Trim the base from each lettuce, wash and dry the leaves, then roughly shred them. Cut the tomatoes into wedges. Finely slice the spring onions (using both white and green parts). Cut the avocados in half lengthways, remove the stones and peel. Cut crosswise into slices, and squeeze with lemon juice. Crumble the feta cheese with your fingers and toss it with the dried oregano and mint. Pick the leaves off the parsley and set aside.
Whisk the 3 tbsp of lemon juice with the olive oil, sea salt and pepper. In a large bowl, toss the chicken, lettuce, tomatoes and spring onion, then fold through three quarters of the dressing with the avocados (carefully, so the avocados don’t break up). Season with salt and pepper, then scatter over the feta, olives and parsley, and drizzle with the rest of the dressing. Squeeze extra lemon juice on top, and serve with pitta bread if you wish.