For the remoulade: In a medium bowl, whisk together the mayonnaise, parsley, lemon juice, Creole Mustard, garlic, Creole seasoning, paprika, tomato paste and celery. Season with salt and pepper and set aside until ready to serve.
For the po'boys: Mix together the flour, cornmeal, Creole seasoning, pepper and salt. Dredge the oysters through the flour mixture. Shake off any excess flour, and rest the oysters on a plate.
Fill a cast-iron skillet or heavy-bottomed pan with 1-inch oil. Heat over medium-high heat until a deep-frying thermometer inserted in the oil reaches 375 degrees F. Fry the oysters until golden and crispy, turning as they cook to ensure even color, about 2 minutes. Drain on a paper-towel-lined plate.
Spread each side the rolls with some remoulade. Divide the oysters between the rolls. Top with some lettuce and tomatoes to serve.
In a medium saucepan add 1/2 to 1 cup water, the vinegar, mustard seeds and garlic. Bring to a boil and reduce the heat to a simmer. Cook at a simmer for 30 minutes.
Remove the saucepan from the heat and, while still hot, mix in the whole-grain mustard, honey, mustard, horseradish, sugar, salt and cayenne. Refrigerate until cool.
For a smooth mustard, blend in a food processor before serving.
Store any leftover mustard in an airtight container in the refrigerator and use on sandwiches, hot dogs or other dishes.