Cut the catfish into 2-inch strips and pat dry. Stir together the flour and 1 tablespoon of the Cajun seasoning with 1/4 teaspoon each salt and pepper. Whisk another tablespoon of the Cajun seasoning into the buttermilk. Pulse the tortillas in a food processor until finely ground with a third tablespoon of the Cajun seasoning.
Add the vegetable oil to large, heavy-bottomed pot and bring to 175°C over medium heat.
Toss the catfish with the seasoned flour, then place the catfish in a sieve and shake off any excess flour. Divide the catfish strips into two batches. Dip the first batch in the buttermilk, letting the excess drip off, and then dredge in the tortillas. Fry the catfish until golden, about 5 minutes. Drain on paper towels. Dredge and fry the second batch of catfish in the same manner.
Stir together the mayonnaise and the remaining tablespoon Cajun seasoning. Spread evenly on the rolls. Divide the iceberg and tomato slices among the rolls. Top with the catfish. Serve with hot sauce.