Bring steaks to room temperature. Take the fillet steaks out of the fridge 30–45 minutes before cooking. Pat them dry thoroughly with paper towels. Dry surfaces sear better.

Preheat the air fryer. Set it to 400°F (200°C) for at least 5 minutes. A hot basket kick-starts browning.

Season generously. Rub steaks lightly with oil. Season all sides with kosher salt and black pepper. Add a light sprinkle of garlic powder if you like. Press seasoning in so it adheres.

Arrange in the basket. Place steaks in a single layer with space around them. Avoid stacking or crowding so air can circulate.

Cook the first side. Air fry 5–6 minutes for medium-rare on 1.5–2-inch-thick steaks. For thinner steaks, start checking at 4 minutes.

Flip and finish. Turn the steaks and cook another 4–6 minutes, depending on thickness and desired doneness. Target internal temperatures: 120–125°F (49–52°C) for rare, 130–135°F (54–57°C) for medium-rare, 140–145°F (60–63°C) for medium. Use an instant-read thermometer for accuracy.

Add butter and herbs. During the last 1–2 minutes, place a pat of butter and a sprig of thyme or rosemary on each steak. The butter melts and bastes the surface.

Rest the steaks. Transfer to a plate and tent loosely with foil for 5–7 minutes. Resting preserves juices and evens out the temperature.

Finish and serve. Top with a pinch of flaky sea salt. Serve with a simple salad, roasted potatoes, or a peppercorn or chimichurri sauce.