Coat one side of 4 (18-inch) squares of heavy-duty aluminum foil with cooking spray.
Combine lemon-pepper seasoning, salt, and dillweed. Arrange potato slices evenly in centers of coated foil squares. Sprinkle potato with one-third of seasoning mixture. Place onion over potato; sprinkle with one-half of remaining seasoning mixture. Top with carrot slices, and sprinkle with remaining seasoning mixture. Place fish over vegetables. Fold foil over fish and vegetables; crimp edges to seal.
Place foil packets on a baking sheet. Bake at 450° for 30 to 35 minutes or until fish flakes easily when tested with a fork.