Melt the butter in a large skillet over medium-high heat. Add the corn and 1/4 teaspoon at the salt. Cook, stirring, until lightly browned, about 5 minutes. (The corn will make popping sounds as it cooks). Let cool completely.
Combine the onions, green onions, celery, Creole and Dijon mustards, lemon juice, parsley, ketchup, horseradish, cayenne, and the remaining 1/2 teaspoon salt in a food processor or blender and purée. With the machine running, add the egg, then slowly add the oil in a thin, steady stream until the mixture thickens, like a mayonnaise. Transfer to a bowl and fold in the corn. Season to taste if necessary and serve. (The remoulade can be covered, and refrigerated for up to 8 hours.)
Yield: 2 1/4 cups