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Pour the lemon juice into a large bowl and whisk while you slowly add the oil. When all the oil has been added and the mixture starts to thicken, season.
Add the lettuce, tomatoes, parsley, cucumber, chicken pieces and spring onions, and stir well to coat the salad in the dressing.
Put the pitta breads in the toaster until crisp and golden, then chop into chunks. Scatter the toasted pitta pieces over the salad and sprinkle over the sumac. Serve straight away.