Add the sliced onions, chopped peppers on top of the chicken, then add the tin of chopped tomatoes, additional cherry tomatoes if you are using them, tomato puree, red wine vinegar and oregano.
2 red onions, 1 red pepper, 1 yellow pepper, 400 g tin of chopped tomatoes, handful cherry tomatoes (optional), 3-4 tbsp tomato purée, 2 tsp red wine vinegar, 1-2 tsp oregano