Chicken kebab platter

Name: Chicken kebab platter
Ingredients: 1 x 640g pack skinless, chicken thigh fillets; 60g mayonnaise; 3 lemons; 2 tsp tomato purée; 2 tsp dried oregano; 1 tsp Turkish pepper flakes* (Aleppo) or 1⁄2 tsp regular chilli flakes; 3 garlic cloves; 1 red onion, finely sliced; a handful of roughly chopped parsley leaves; 1 x 400g tin chickpeas; 120g tahini; 1⁄4 red cabbage, shredded; 1 Little Gem lettuce, shredded; extra-virgin olive oil to drizzle; about 1⁄2 tsp sumac; warmed flatbreads, to serve