Step 1Blanch the banana leaves, a few at a time, in boiling water for 2 to 3 minutes until slightly softened. Remove to a bowl of cold water, then drain, pat dry and set aside.
Step 2In the bowl of a food processor, combine the chiles, garlic, parsley, cilantro, lime juice and zest, and salt. Process until blended. Add the olive oil in a slow stream and process until combined; you'll have about 1 cup of sauce.
Step 3Place a leaf or leaf section on the work surface, rib side up. Center one halibut fillet on the leaf.
Step 4Add 1 tablespoon of sauce to the top of the fish and turn to coat the fillet. Wrap the leaf around the fish, turn the packet over, and use a second leaf to wrap the fish again. Repeat with the remaining fillets.
Step 5Grill the packets over medium heat, turning once, until the leaves are crisp and brown but not burned through, about 20 minutes. If you're in doubt, open one packet to test that the fish is flaky and cooked through. Serve immediately, with rice and the remaining sauce passed at the table.