Preheat the oven to 450 degrees F. Line a large baking sheet with foil or parchment.
In a food processor, whizz up the lime juice, cilantro, mint, fish sauce, sesame oil, brown sugar, red pepper flakes, pepper, and oil.
Cut 4 to 5 slits on each side of the fish, almost down to the bone.
Divide the cilantro paste in half. Rub 1 portion of the paste all over fish, especially into the slits and the cavity of the fish. Reserve the other half for serving.
Take a large piece of foil and form a loose ball. Stuff it into the cavity so that you can make the fish "sit" on the baking sheet, belly facing down, ridge of the fish's back facing the ceiling. It'll look as if the fish is swimming on the baking sheet!
Bake until the eyes turn solid white and the flesh flakes easily, 15 to 20 minutes, depending on the size of the fish.
Taste the leftover marinade, and add juice from the other half of the lime. Serve marinade on the side with the fish.