Preheat oven to 450°.
Coat 1 side of 4 (18-inch) squares of heavy-duty aluminum foil with cooking spray.
Combine seasoning, salt, and dillweed. Layer potato, onion, and carrot slices evenly in centers of coated foil squares. Sprinkle evenly with seasoning mixture. Place fish over vegetables. Fold foil over fish and vegetables; crimp edges to seal. Place packets on a baking sheet. Bake at 450° for 30 minutes. Serve immediately.
Tip: Toss together a salad while the fish bakes and dinner is ready as soon as the fish comes out of the oven.