1. Prepare grill to medium-high heat.
2. Boil orange juice in a small saucepan 15 minutes or until reduced to 1/4 cup. Transfer to a small bowl; stir in soy sauce.
3. Place pineapple and onion on a grill rack coated with cooking spray. Grill 5 minutes on each side or until the onion is tender. Remove from grill; let stand 5 minutes. Coarsely chop, and transfer to a bowl.
4. Brush both sides of fish evenly with oil; sprinkle evenly with 1/4 teaspoon salt and 1/8 teaspoon pepper. Place fish on a grill rack coated with cooking spray; brush with half of juice mixture. Grill 3 minutes; turn fish over. Brush fish with remaining juice mixture; grill 3 minutes or until desired degree of doneness.
5. Add the remaining 1/8 teaspoon salt, remaining 1/8 teaspoon pepper, basil, and next 3 ingredients (through serrano) to pineapple mixture; toss. Serve with fish; garnish with mint sprigs, if desired.