Preheat oven to 380°F.
Wipe the turmeric leaf down gently with a damp towel.
Arrange flat on a work surface with the shiny side facing down.
Brush the leaf with the ají dulce kosho mixture for the length of the fillet in the broader part of the leaf, avoiding excess oil.
Arrange fish on top of the brushed area, proceed to season with kosher salt and brush with more ají dulce kosho, making sure to coat it evenly. Top with extra cubed peppers present in the mixture using a slotted spoon.
Fold the ends first and wrap the sides of the leaf around the fish, making a tidy little packet with the folds facing down.
Arrange papillote on a metal sheet pan with the folded side down (the weight of the fish will hold everything together) and allow to marinate for 10 minutes.
Bake the papillote for 13 minutes (if the fillet is thick, bake for 16 minutes)
Serve immediately — instruct your guests to slowly open the papillote with care with their noses over the dish, allowing the aromatic steam to be part of the experience.