1. Prepare Creole Seasoning: Combine all ingredients in a medium bowl, stirring well.
2. Prepare Catfish: Preheat oven to 300°. Pour oil to a depth of 3 inches into a 4- to 5-qt. Dutch oven; heat over medium-high heat to 350°.
3. Meanwhile, combine cornmeal, masa harina, and 3 tablespoons Creole Seasoning in a large bowl, reserving remaining seasoning for another use. Stir seasoning mixture well.
4. Whisk eggs in a large bowl; whisk in milk and mustard. Dip fish in egg mixture; dredge in cornmeal mixture, shaking off excess.
5. Fry fish, 8 pieces at a time, 2 minutes on each side. Drain fish on paper towels. Transfer fish to a baking sheet lined with parchment paper. Place in oven; keep warm at 300° until ready to serve. Serve with pickled vegetables and sliced tomatoes.