In a large saucepan, heat peanut oil over medium heat. Add chopped onion and sauté until it's soft and translucent, about 3 to 5 minutes.
Incorporate the diced red bell pepper and continue cooking for an additional 3 to 5 minutes until it's tender.
Mix in the curry paste and let it cook for about 1 minute to release its flavors.
Pour in the coconut milk and gently bring the mixture to a simmer.
Once the coconut milk is simmering, add cubed monkfish to the pan and allow it to simmer for 7 to 10 minutes until the fish is cooked through and no longer opaque.
Finish by stirring in fish sauce, fresh lime juice, and chopped cilantro just before serving.