Green goddess avocado salad

Name: Green goddess avocado salad
Ingredients: 2 tbsp neutral-tasting oil (optional); 4 chicken breasts, or 4 cooked chicken breasts, sliced; 2 sticks celery, finely sliced; 1 round lettuce, leaves separated, or 2 Little Gem lettuces; 200g radishes, thinly sliced; 1 green apple, cored and finely sliced; 1 cucumber, finely sliced; 1 large avocado, pitted and peeled, roughly chopped; 200g Greek-style yogurt; 15g chives, roughly chopped; 15g dill, roughly chopped; 15g parsley, roughly chopped; 3 spring onions, roughly chopped; 4 tbsp rice vinegar; 3 tbsp extra virgin olive oil; 1 large garlic clove, crushed; 1 lemon, juiced (reserve the zest, to serve)