Heat peanut oil in a large sauce pan over medium heat. Stir in chopped onion and cook until softened and translucent, 3 to 5 minutes. Add red capsicum and continue to cook for 3 to 5 more minutes, until softened. Stir in the curry paste and cook for 1 minute. Pour in the coconut milk and slowly bring to a simmer.
Once coconut milk begins to simmer stir in cubed fish and simmer 7 to 10 minutes or until the fish is firm and the centre is no longer opaque. Stir in fish sauce, lime juice and coriander before serving.