In a large pot, heat oil and cook onions for a few minutes until soft. Add curry paste and continue to cook for another minute.
Add beans, stock, basil, chili pepper, lime juice and zest. Bring to a boil, reduce and simmer for 15 minutes.
Remove basil and chili pepper, then run soup through a food processor or immersion blender until smooth. Return to pot and add coconut milk and fish sauce, bring back to a simmer.