Catfish Sandwich with Remoulade

Name: Catfish Sandwich with Remoulade
Ingredients: 2/3 cup ketchup; 1/3 cup mayo or mild aioli; 3 tablespoons red wine vinegar; 1 tablespoon capers, drained and minced; 2 teaspoons prepared horseradish; 2 tablespoons finely minced celery; 2 tablespoons finely minced red onion; 4 anchovies, rinsed and minced; 1 teaspoon dry English mustard; 1 teaspoon paprika; 1/2 teaspoon kosher salt/ pepper blend; 1/8 teaspoon ground cayenne pepper; 1 quart vegetable oil; Four 6-inch brioche or kaiser buns; Butter, for toasting; Four 5-ounce catfish fillets; 1 cup all-purpose flour seasoned with 1 teaspoon Cajun spice; 2 cups egg wash (equal parts milk and egg); 1 cup coarse cornmeal; 1 cup shredded romaine; 8 slices tomatoes; 1 cup thinly sliced red onion