Coat corn and onion with cooking spray; place on grill rack coated with cooking spray. Cover and grill corn 20 minutes, turning 3 times. Grill onion 10 minutes or until tender, turning once.
Brush fish with 1 tablespoon oil; sprinkle with 1/2 teaspoon salt and 1/2 teaspoon pepper. Place fish on grill rack; grill 3 minutes on each side or until desired degree of doneness.
Cut kernels from ears of corn in a medium bowl. Add onion, remaining 1/2 tablespoon oil, remaining 1/4 teaspoon salt, remaining 1/4 teaspoon pepper, basil, and vinegar, stirring to combine. Serve fish with corn salad.