Heat a little olive oil in a frying pan over a high heat. Season the beef and sear it on all sides for about 8–10 minutes, until nicely browned. Transfer to a plate.
Meanwhile, cut the fennel into long wedges and the onion into wedges.
Toss the fennel and onion with a little olive oil, the caraway seeds and some freshly ground black pepper. Arrange them in the air-fryer basket or a small roasting dish.
Pour over the white wine or dry sherry.
Brush the beef generously with the Dijon mustard and place it on top of the fennel and onion.
Roast in a preheated air fryer or oven at 180°C for about 30 minutes, depending on the size and shape of the joint and your preferred doneness.
Baste the fennel with the wine and cooking juices a few times during cooking so that it doesn't dry out. If using an air fryer, turn the beef over at least once for more even cooking.
For medium-well beef, aim for an internal temperature of around 58–60°C; for well done, aim for approximately 66–70°C. The temperature will continue to rise slightly while the meat rests.
Transfer the beef to a plate, loosely cover and rest for about 10 minutes.
Slice the beef and serve with the roasted fennel, onion and the cooking juices as a simple jus.