Place the oil in a saucepan over low heat, add the onion
and cook for 5 minutes until softened slightly. Add all the
other ingredients and bring to the boil. Reduce heat to medium
and cook for 5 minutes, stirring, until thickened slightly.
Set aside to cool slightly.
Place in a blender and blend until smooth. Store in an
airtight container in the fridge for up to 2 weeks.