If using raw chicken, heat the oil in a frying pan over a medium-high heat. Season the chicken all over and fry for 7-8 mins each side, covering with a lid for the final 5 mins until cooked through. Set aside to rest for 10 mins, then slice.
For the dressing, tip the avocado, yogurt, chives, dill, parsley, spring onions, rice vinegar, olive oil, garlic and 1 tbsp of the lemon juice into a blender or food processor. Blitz until finely chopped and emulsified. Season to taste, adding more lemon juice if you like. Add cold water 1 tbsp at a time until the dressing has a drizzling consistency.
Stir 4 tbsp of the dressing together with the chicken slices. Toss the celery, lettuce, radishes, apple and cucumber together and arrange on a serving platter. Dot the chopped avocado and as much of remaining dressing over the top as you like, along with the chicken. Finely grate a little lemon zest over and grind over some black pepper to serve.
