Cajun Steak and Grits

Name: Cajun Steak and Grits
Ingredients: 1 1/2 pounds flank steak; Kosher salt and freshly ground pepper; 2 tablespoons Cajun seasoning; 2 tablespoons extra-virgin olive oil; 1 cup quick-cooking grits; 4 ounces extra-sharp cheddar cheese, shredded (about 1 cup); 1/2 cup whole milk; 3 tablespoons unsalted butter; 1 onion, diced; 1 red bell pepper, diced; 2 stalks celery, diced, plus celery leaves for topping; 3 tablespoons apple cider vinegar