Crush 1 garlic clove and mix with 3 tbsp yogurt, the lemon juice, 1 tsp tomato purée, half the spices, and some seasoning in a medium bowl. Add the chicken, toss, cover and chill in the fridge for at least 1 hr or overnight.
Make the chilli sauce. Put 100g of the tomatoes, the whole red chilli, the remaining spices, garlic clove and tomato purée, plus 1 tbsp olive oil, half the red onion and the pomegranate molasses in a food processor. Add some seasoning and pulse until puréed, then set aside.
Heat oven to 200C/180C fan/gas 6, and heat a griddle pan. Thread the chicken onto metal skewers or wooden ones that have been soaked, then wipe off the excess marinade. Skewer the remaining tomatoes on a separate skewer. Lightly oil the griddle with the remaining olive oil and brown the chicken on both sides, then transfer to a baking tray and bake in the oven for 10 mins, or until cooked through. Meanwhile, griddle the tomatoes for 1-2 mins, turning halfway through so they have griddle marks on both sides.
Serve the chicken and tomatoes on flatbreads with the remaining red onion, pickled chillis, the remaining Greek yogurt, parsley and coriander with the chilli sauce spooned over.