Remove the beef tenderloin from the refrigerator and let it sit at room temperature for 30 to 45 minutes. This ensures even cooking throughout the roast.
In a small bowl, combine the olive oil, minced garlic, rosemary, thyme, kosher salt, black pepper, and onion powder into a paste. Rub this mixture generously over the entire surface of the tenderloin, pressing the herbs into the meat.
Preheat the air fryer to 400 degrees F for 3 minutes.
Place the seasoned tenderloin in the air fryer basket. Cook at 400 degrees F for 20 to 25 minutes for medium-rare, turning the roast over halfway through cooking. For medium, add 3 to 5 more minutes.
Use an instant-read thermometer inserted into the thickest part. Pull at 130 degrees F for medium-rare or 140 degrees F for medium. While the tenderloin cooks, mix the softened butter with the dijon mustard until smooth. Set aside.
Transfer the cooked tenderloin to a cutting board and immediately spread the mustard butter over the top while the meat is still hot. Tent loosely with foil and rest for 10 minutes.
Remove the kitchen twine, slice the tenderloin into 1/2 inch thick medallions, and serve. Spoon any collected juices from the cutting board over the slices.