In a medium bowl, whisk together mustard, paprika, TABASCO® Sauce, salt, and pepper. Beat in vinegar and then, whisking constantly, add oil in a slow, thin stream, continuing to beat until sauce is thick and smooth. Stir in onions, celery and parsley and mix well. Cover bowl and let sauce stand at least 2 hours before serving to allow flavors to blend.
Makes 2 cups.