Mince garlic and, wearing protective gloves, mince serranos. In a blender purée all sauce ingredients until smooth.
If using banana leaves, cut away ribs and in a large bowl of hot water soften leaves 1 minute to help keep them from splitting.
Cut six 8-inch squares from leaves. On a work surface arrange 1 leaf square and pat dry.
Spread 1 teaspoon herb sauce down middle of square to equal length of a piece of fish and arrange piece, skin side down, on sauce.
Spread 1 tablespoon sauce over piece. Fold top and bottom edges of leaf over piece and fold sides of leaf over piece, enclosing it. Arrange packet, seam sides down, in a bamboo steamer or on a large steamer rack. Make 5 more packets in same manner and arrange in steamer or on rack.
Steam packets over boiling water, covered, until fish is just cooked through, about 8 minutes.
