In a large Dutch oven, pour oil to a depth of 4 inches, and heat over medium heat until a deep-fry thermometer registers 400° to 425°.
In a large bowl, combine mustard, hot sauce, and ½ tablespoon Cajun seasoning.
Coat seafood with the mustard mixture. (Do not dredge shrimp into mixture more than 15 minutes before frying, as the vinegar in the mustard can make the shrimp tough.)
In a medium bowl, combine corn flour and remaining 3 tablespoons Cajun seasoning. (I add 1 Tablespoon salt)
Coat seafood in mustard mixture and then dredge in corn flour mixture, coating completely and shaking off any excess. Place seafood on prepared wire rack up to 30 minutes before cooking.
In batches, fry seafood, stirring after 40 seconds, until golden brown. (shrimp 2 to 4 minutes) Remove from oil, and let drain on a cooling rack. Squeeze lemon juice over seafood, and sprinkle with salt.
