In a pan on high heat grill the bell pepper, garlic, chillies (if using) and brown onion/spring onion for 2-3 min on each side until slightly blackened.
Transfer to a bowl with a lid or cover with cling film to steam for a few minutes.
Add everything into the food processor and pulse until small fine chunks for a topping on meat or seafood, not pureed. This sauce makes an excellent marinade also and is great to keep in an airtight container in the fridge for up to a week.