Melt 2 tablespoons butter in a medium nonstick skillet over medium-high heat. Add onion and carrot, and sauté 3 minutes. Add celery, leeks, garlic, salt, and 1/2 teaspoon Creole seasoning; sauté 5 minutes. Add wine, lemon juice, and thyme. Cook 2 minutes or until slightly thickened.
Cut 6 (18- x 12-inch) rectangles of foil. Brush center of each sheet with olive oil. Place 1 fillet in center of each sheet, and sprinkle evenly with remaining 2 teaspoons Creole seasoning. Divide onion mixture evenly over fillets. Top each fillet with 1 tablespoon remaining butter. Fold foil in half over each fillet to form a packet; crimp edges to tightly seal.
Grill packets on grill rack, covered with grill lid, over medium heat (300° to 350°) 12 minutes.