1. Prepare outdoor grill for barbecuing.
2. With vegetable peeler or fish scaler, remove any scales from fish. Cut 3 or 4 crosswise slashes, about 1/2 inch deep, on each side of fish. Rinse fish; pat dry with paper towels.
3. In 10-inch skillet over medium heat, melt 6 tablespoons butter or margarine (3/4 stick). Add mushrooms and green onions; cook until vegetables are tender. Remove from heat; stir in bread crumbs, tomato, 1/2 teaspoon salt, 1/2 tea spoon thyme, and 1/4 teaspoon pepper.
4. Place fish on double thickness of heavy-duty foil; spoon mushroom mixture into fish cavity; secure with toothpicks. Sprinkle fish with lemon juice, 1/2 teaspoon salt, 1/4 teaspoon thyme, and 1/8 teaspoon pepper; dot with 2 tablespoons butter or margarine 1/4 stick). Wrap fish tightly with foil, being careful seam is folded several times to seal in juices;
5. Place foil-wrapped fish on grill over medium coals; cook 30 minutes, turning once, until fish flakes easily when tested with fork. Place fish on warm large platter. Discard toothpicks; pour juices in foil over fish.