Place first 7 ingredients in a large, heavy-duty zip-top plastic bag. Add fish; seal and shake gently. Chill 1 hour.
Pour oil to depth of 3/4 inch in a large, heavy-duty skillet; heat to 350°.
Combine fish-fry mix and cornstarch in a separate large, heavy-duty zip-top plastic bag. Remove fish from milk mixture. Place in bag in batches, and shake to coat.
Fry fish, in batches, in hot oil 2 to 3 minutes on each side or until golden. Drain on paper towels. Separate onion slices into rings, and place over fish. Serve immediately with Creole Tartar Sauce.
*We used Zatarain's fish-fry mix for testing.