Rinse fish fillets under cold water, cut into 2-inch wide strips; place in a large bowl.
In a large bowl or tupperware container, combine buttermilk, yellow mustard, Tabasco Sauce, lemon juice, and 2 tablespoons cajun seasoning. Whisk together well.
Cover fish with buttermilk mixture and mix with hands; cover and refrigerate: let soak for 30 minutes to an hour before frying.
In a large zip top bag, combine flour, cornmeal, cajun seasoning, garlic powder and black pepper.
Heat oil in a large iron skillet over medium-high heat to about 400 degrees or in a deep fryer.
Meanwhile drain off any excess buttermilk mixture from the fish. Drop 3 to 4 pieces into the cornmeal mixture and coat each piece well.
Fry fish pieces for 3 to 4 minutes per side, or until golden brown.
Remove from hot oil and drain on paper towels or brown paper bags.
Serve hot with tarter sauce.