Thaw fish, if frozen. Rinse fish; pat dry with paper towels. Cut into 4 serving-size pieces. Refrigerate fish until needed. In a small bowl, whisk together white balsamic vinegar, mustard, the water, 2 teaspoons of the olive oil, and the 1 clove garlic. Set aside.

Trim roots and cut off green tops of leeks; remove 1 or 2 outer white layers. Wash well (if necessary, cut a 1-inch slit from bottom end to help separate layers for easier washing). Drain. In a medium saucepan, combine leeks and a small amount of water. Bring to boiling; reduce heat. Cover and simmer for 3 minutes. Drain. Pat dry. Brush with 1 teaspoon of the remaining olive oil.

In a small bowl, stir together the remaining 1 teaspoon olive oil, the 3 cloves garlic, the salt, and pepper. Spread evenly over fish; rub in with your fingers.

Place fish and leeks on the greased rack of an uncovered grill directly over medium coals. Grill for 8 to 12 minutes or until fish flakes easily when tested with a fork, gently turning fish and leeks once. Slice leeks into 1/2-inch pieces. Divide fish and leeks among four shallow bowls. Drizzle with vinegar mixture. If desired, garnish with tomato halves and chives. Makes 4 servings.