1. Place onion in a small bowl; add water to cover. Let stand for 30 minutes. Drain. Set aside.
2. Arrange fish fillets in a single layer on a large plate lined with paper towels. Let stand, uncovered, in refrigerator for 1 hour (to allow surface to dry).
3. Preheat grill to medium-high heat.
4. Place potatoes in a large saucepan. Cover with water to 2 inches above potatoes; bring to a boil. Reduce heat; simmer 15 minutes or until tender. Drain; cool slightly. Cut potatoes in half. Combine potatoes, thyme, 1 tablespoon olive oil, 1/2 teaspoon salt, and 1/8 teaspoon pepper in a bowl, tossing to coat. Thread potatoes onto 4 (6-inch) skewers with cut sides facing out.
5. Combine onion, remaining 1 tablespoon oil, 1/8 teaspoon salt, 1/8 teaspoon pepper, tomatoes, mint, capers, vinegar, and mustard in a bowl; toss gently. Set aside.
6. Sprinkle remaining 3/8 teaspoon salt and 1/4 teaspoon pepper evenly over fish. Place fish and potatoes, cut sides down, on grill rack coated with cooking spray. Cover and grill potatoes 6 minutes without turning. Grill fish 3 to 4 minutes on each side or until fish flakes easily when tested with a fork or until desired degree of doneness. Serve fish with potatoes; top with tomato-onion relish.
Sustainable Choice: Arctic char is a sound choice; you can also use wild Alaskan salmon in its place.