Heat oven to 350°F. Cut 4 (18x18-inch) sheets of heavy-duty foil.
Place 1 fillet on center of each sheet. Top each with 1/4 each of the onion, salsa, roasted red peppers, olives, olive oil, nopales and black pepper. Wrap packets securely using double-fold seals, leaving a small opening so steam can escape during baking.
Bake 30 to 35 minutes or until fish flakes easily with fork. Sprinkle with cilantro.