1. Prepare grill to medium-high heat.
2. Brush mango and onion with 1 teaspoon oil. Place mango and onion on grill rack coated with cooking spray; cover and grill 3 minutes on each side or until tender. Chop onion and mango. Combine onion, mango, avocado, mint, juice, 1/4 teaspoon salt, and 1/8 teaspoon pepper in a medium bowl.
3. Brush fish with remaining 1 teaspoon oil; sprinkle with 1/4 teaspoon salt and 1/8 teaspoon pepper. Place fish on grill rack; grill 4 minutes on each side or until fish flakes easily when tested with a fork or until desired degree of doneness. Serve with mango mixture. Garnish with mint sprigs, if desired.
Orange-scented couscous: Bring 3/4 cup fresh orange juice, 1/2 cup water, and 1 tablespoon olive oil to a boil in a saucepan. Stir in 1 cup uncooked couscous, 1/4 teaspoon salt, and 1/8 teaspoon black pepper. Remove from heat, cover, and let stand 5 minutes. Fluff with a fork.
Can't find snapper? Substitute any firm, white fish instead, like orange roughy or mahi-mahi.