Combine first 4 ingredients in a shallow dish; set aside.
Stir together buttermilk and egg in a bowl; set aside.
Combine cornmeal, garlic powder, and thyme in a zip-top freezer bag.
Dredge catfish in flour mixture, and dip in buttermilk mixture, allowing excess to drip off. Place catfish in cornmeal mixture; seal bag, and shake to coat.
Pour oil to a depth of 1 1/2 inches into a large cast-iron skillet; heat to 360°. Fry catfish, in batches, 3 minutes on each side or until golden. Drain on wire racks over paper towels, and serve immediately.