1. Stir together mayonnaise, vinegar, 1 Tbsp. Creole mustard, and 1 tsp. Cajun seasoning in a medium bowl until blended. Stir in coleslaw. Cover and chill.
2. Meanwhile, brush steaks with remaining 1 Tbsp. mustard, and sprinkle with salt and remaining 1/2 tsp. Cajun seasoning. Heat a grill pan over medium-high heat. Cook steaks in pan 6 minutes on each side or to desired degree of doneness. Cook onion 5 minutes on each side. Serve steak and onions immediately with coleslaw.
Note: We tested with McCormick Cajun Seasoning.