2) Meanwhile, cook the onions in 3 tbsp oil in a large saucepan over a medium-low heat for 10 to 15 minutes, until translucent. Add the garlic and cook for 1 more minute. Add the peppers, chilli powder, cumin, red pepper flakes, cayenne, and salt. Cook for 1 minute.
3) Crush the tomatoes by hand or in batches in a food processor fitted with a steel blade (pulse 6 to 8 times). Add to the pan with the basil, bring to a boil and then reduce the heat and simmer, uncovered, for 30 minutes, stirring occasionally.
4) Remove the chicken from the oven and leave to cool slightly. Separate the meat from the bones and skin and cut it into 2cm chunks. Add to the chilli and simmer, uncovered, for another 20 minutes. Serve with the toppings, or refrigerate and reheat gently before serving.