Preheat air fryer to 390°F (200°C).
Mix sea salt, rosemary, and oil on a plate.
Blot roast with paper towels. Place on plate and turn so oil and herb mixture coats outside.
If using onion, peel, halve, and place halves in air fryer basket.
Place roast in air fryer basket.
Set air fryer to 390°F (200°C) to cook beef for 5 minutes.
When air fryer is done, turn to 360°F (180°C). Turn roast halfway through cooking if your air fryer requires it [see Note 1].
Cook for an additional 30 minutes for rare beef. It is advisable to monitor the temperature with a meat thermometer to ensure it is cooked to your liking. Check early and add in 5-minute intervals if you prefer it well done.
Remove the roast from the fryer, cover with foil and let it rest for at least 10 minutes before serving. This allows the meat to finish cooking and the juices to reabsorb.
Carve the roast thinly against the grain and serve with roasted or steamed vegetables, whole grain mustard and jus.