Heat vegetable oil to 350 degrees in a large, heavy bottom saucepan or cast iron pot. Allow for 2 inches of oil for proper frying, adding more oil if necessary.
Cut each catfish filet into 4 to 6 pieces equal in size.
Place catfish pieces in a single layer on a baking sheet.
Season both sides with salt, red pepper, and granulated garlic, to taste.
Add mustard and toss to coat each piece.
Combine cornmeal and flour in large mixing bowl.
Season cornmeal and flour mixture with salt, red pepper, and granulated garlic.
Add catfish pieces to the mixture and toss.
Shake off excess corn meal and set aside.
Fry catfish pieces in batches, paying attention not to crowd pan.
When pieces begin to float and brown on the edges, turn each piece.
Continue frying until golden brown.
Remove the catfish from the oil and place on cooling rack.