Preheat your air fryer to 400°F (200°C).
In a small bowl, mix together olive oil, Dijon mustard, minced garlic, dried thyme, dried rosemary, salt, and black pepper.
Pat the steaks dry with a paper towel. Brush the tops of the steaks with the herb marinade, ensuring they are well-coated. Reserve one teaspoon of the seasoning for the herb butter.
Place the steaks in the air fryer basket, leaving some space between them for even cooking.
Air fry the steaks for 10-12 minutes for medium-rare, adjusting the time based on your desired level of doneness. For a perfect medium-rare, the internal temperature should reach around 130°F (54°C).
While the steaks are cooking, take the butter, reserved herb mixture and the parsley and mix together in a small bowl until well combined. Place in the refrigerator until serving time.
Once done, remove the steaks from the air fryer and let them rest for a few minutes before serving.
Serve the sirloin steaks topped with the herb butter alongside your favorite sides, such as roasted potatoes or roasted green beans.