Coriander and cumin seeds can be bought already ground. But for better flavor and shelf life, buy the seeds and grind them with a mortar and pestle or spice grinder (clean coffee grinders work, too).
Slice fish fillets horizontally (not all the way through) to create pockets. Sprinkle with salt and 4 tablespoons lemon juice.
Pulse remaining 2 tablespoons lemon juice and next 4 ingredients in a food processor until they become a paste. Rub paste on both sides of each fillet, filling pockets. Marinate in refrigerator 30 minutes.
Place one fish fillet in center of each banana leaf. Fold in left side of leaf, then right side, and roll to form a packet. Place on a steamer rack or basket, seam-side down, to keep leaves from opening. Repeat with remaining fillets and banana leaves.
Fill a large Dutch oven with 1 inch of water; bring to boil. Lower steamer rack into pan slowly. Cover and steam fish packets 8 minutes or until fish flakes easily with a fork. Remove from steamer; unwrap each packet. Arrange on a platter, and garnish, if desired. Serve immediately.