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 bell 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 center is no longer opaque. Stir in fish sauce, lime juice, and cilantro before serving.