Preheat the oven to gas 6, 200°C, fan 180°C. Heat 2tbsp oil in a large frying pan over a medium heat. Add the onions and peppers and fry for 3-4 minutes or until slightly softened. Add the garlic and fry for 1 more minute. Tip the onion and pepper mix into a large ovenproof dish, set aside.
Drizzle the chicken pieces with the remaining olive oil, season well, then fry in the frying pan until golden-brown all over. Put the chicken on top of the peppers and onions in the ovenproof dish.
Pour the tinned cherry tomatoes, stock and chilli flakes (if using) into a pan, season well and warm through over a low heat. Pour the sauce around the chicken pieces and vegetables, then add the rosemary sprigs and bake, covered with foil, for 30 minutes.
Remove the foil, scatter with the black olives and bake for another 15 minutes or until the chicken is cooked. Cut into a thick section of the chicken to ensure it is cooked through with no pink showing.
Scatter with the chopped parsley and extra chopped rosemary, then serve.
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