Peel and clean shrimp. In a large bowl, combine buttermilk, 3 tbsp Cajun seasoning, ground mustard, and hot sauce. Add shrimp, cover, and refrigerate for 30 minutes.
For the remoulade, combine mayo, mustard, lemon juice, Worcestershire sauce, hot sauce, horseradish, and Cajun seasoning. Whisk until smooth and set aside.
Preheat oil to 350°F. In a bowl, mix flour, cornmeal, and remaining Cajun seasoning.
Remove shrimp from marinade, coat in flour mixture, and set aside. Fry shrimp in batches for 3-4 minutes or until crispy. Drain on a rack or paper towels.
Toast French bread, then spread remoulade, layer with lettuce, tomato, and shrimp. Add more remoulade or hot sauce if desired.
Serve hot and enjoy!