Using a knife, score the steak on both sides with shallow cuts across the grain in a criss-cross pattern. Season both sides with salt and black pepper.
In a 2-cup measuring cup, whisk together the red wine, olive oil, Worcestershire sauce, garlic, Dijon, and rosemary. Pour the mixture into a plastic zipper bag, then place the steak in the bag and massage the marinade into the meat. Refrigerate for 4-8 hours.
Remove the steak from the bag and place it on a cutting board, then pour the marinade in a saucepan and set it aside.
Pat the steak dry with paper towels. Let it rest and come to room temperature for 30 minutes.
Preheat the air fryer to 400 degrees F. Place the steak inside, and cook for 5-7 minutes on each side, flipping once. (5 mins each side for 130 degrees/med-rare)
Remove the steak and allow it to rest on a cutting board for 10 minutes, then slice thinly against the grain.
While the steak rests, make the sauce (optional). Add beef broth to the saucepan with the marinade. Bring it to a boil over medium-high heat, then reduce the heat and simmer until the sauce thickens. Stir in the butter, taste, and season if necessary.
Notes
How to Reheat Flank Steak in the Air Fryer:
Preheat the air fryer to 400 degrees F. Spray the fryer basket with cooking spray, place the steak inside, and cook for 5-7 minutes until warmed through.
Remove the steak and allow it to rest on a cutting board for 10 minutes, then slice thinly against the grain.