Heat oil in a large frying pan over medium heat. Stir in onion, and cook until softened, 3 to 5 minutes. Add red pepper 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 monkfish, 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 fresh coriander before serving.