Thaw fish, if frozen. Rinse fish; pat dry with paper towels.
On a cutting board, spread each fish open, skin side down.
In a small bowl stir together melted butter and lemon juice. Set half of the butter mixture aside. Brush remaining half of the butter mixture over fish.
In a small bowl stir together onion, rosemary, lemon peel, salt, and pepper. Sprinkle onion mixture over fish.
Fold fish closed. Place fish on a greased 15x10x1-inch baking pan.
Bake in a 450 degree F oven for 15 minutes or until fish begins to flake when tested with a fork.
If desired, sprinkle fish with parsley. Serve fish with lemon wedges and reserved butter mixture.