How to Make It
Preheat oven to 400°F. Heat a large cast-iron skillet over high. Add orange slices to pan; sear until partly blackened, about 2 minutes on each side. Remove orange slices from pan.
Coat fish fillets with oil. Stir together salt, coriander, cinnamon, red pepper, ginger, and nutmeg; sprinkle evenly over tops of fish fillets. Place 3 orange slices, slightly overlapping, in center of each banana leaf piece. Place 1 fish fillet, seasoned side up, on each set of orange slices. Fold each leaf piece to enclose fish. Place packets, folded side down, on a baking sheet. Bake at 400°F until fish is done, about 20 minutes. Serve in packets, or unwrap and transfer fillets and orange slices with a spatula onto plates. Garnish with cilantro, if desired.