Preheat your 5- or 6-quart air fryer to 350°F for 5 minutes.

Tie the beef top sirloin roast at ¾-inch intervals with kitchen string to form a compact shape.

Brush the roast with 2 teaspoons of olive oil and season it.

Carefully pull out the air fryer pan and basket and place the beef roast and pearl onions in the basket.

Slide the pan and basket back into the appliance.

Keep the temperature set at 350°F and set the timer for 45 minutes.

Cook, turning the roast halfway through cooking time, until it reaches your desired level of doneness (see air fryer cooking times below).

Transfer the cooked beef to a tray and brush it with the whole-grain mustard.

Cover the beef loosely with foil and let it rest for 20 minutes.

While the beef is resting, brush the butternut squash with the remaining olive oil, scatter thyme leaves, and season it well.

Place the seasoned squash in the air fryer basket.

Keep the temperature set at 350°F and set the timer for 20 minutes, or until the squash is tender.

Serve the sliced beef with the roasted pearl onions, butternut squash, and gravy.