1. Preheat oven to 350°.
2. Combine first 6 ingredients in a small bowl; stir in 1/8 teaspoon pepper. Set tartar sauce aside.
3. Combine onion and ice water; soak 20 minutes. Drain on paper towels.
4. Sprinkle remaining 1/4 teaspoon pepper and 1/4 teaspoon salt evenly over fish. Place flour in a shallow dish; place egg whites in another dish. Place panko in another. Dredge both sides of fish in flour; dip in egg white, and dredge in panko. Heat a large nonstick skillet over medium-high heat. Add oil to pan; swirl to coat. Add fish to pan; cook 2 minutes on each side or until browned. Place fish on a baking sheet. Bake at 350° for 6 minutes or until fish flakes easily when tested with a fork.
5. Spread about 4 teaspoons mint mixture on bottom half of each bun. Top each with 1 fish fillet. Arrange onion evenly over fish; top with 2 tomato slices and top half of bun.