1. Preheat grill to medium-high (about 450°-500°F).
2. Grill pineapple rounds for 2 to 3 minutes, turning once, until lightly charred and heated through. Remove from grill to cool slightly. Dice and set aside.
3. Season the redfish filets on both sides with 1 tbsp. Cajun seasoning, gently pressing the spices into the fish to adhere. Set aside.
4. Season shrimp with remaining 1 tbsp. Cajun seasoning and set aside.
5. Into an aluminum pan add melted butter. Place seasoned redfish filets into pan and top filets with seasoned shrimp. Scatter onion, ginger, green onion and bird chilies around the pan. Drizzle Jack Daniels over fish and shrimp. Pour barbecue sauce over top fish and onto the vegetables scattered in the pan. Top with grilled pineapple and fire-roasted red pepper. Place pan into grill and close grill lid. Grill-bake for 12 to 15 minutes, checking occasionally that fish isnt sticking, until fish is fully cooked through and shrimp are pink and opaque.
6. Carefully remove pan from grill and transfer redfish filets and shrimp to plates. Serve garnished with watercress with remaining vegetables, grilled pineapple and juices from pan.