To make the glaze, whisk all the ingredients together in a saucepan. Bring to the boil and simmer for 25-30 mins to reduce until sticky. Meanwhile, crush the onions with a pestle and mortar, or blitz in a food processor until you get a fine crumb.
Heat the oven to 180C/160C fan/gas 4. Generously season the beef all over. Heat the oil in a frying pan over a high heat and spend 10 mins searing the beef well on all sides. Remove from the pan (setting the pan aside to cook the carrots later), then sit the beef on a wire rack in a roasting tin. Brush all over with some of the glaze, roast for 10 mins, then turn it over and do the same again, reserving some of the glaze for brushing over at the end. If you want the beef rare and you have a meat thermometer, the core temperature should be 52C. For medium-rare, roast for another 10-15 mins – the core temperature should be 55-60C. Cover the beef and let it rest for 15-20 mins.
Meanwhile, boil the carrots in salted water for 5-7 mins until just tender, then drain. Tip the carrots into the frying pan you used to sear the beef along with the butter and fennel seeds. Fry over a medium heat until golden, then add the spinach, any resting juices from the meat and 1 tsp of the beef glaze. Cook until the spinach has wilted.
To serve, scatter the crispy onions on a tray or plate. Brush the beef again with the glaze and roll it in the onions. Sit on a board, carve into thick slices and serve with the carrots and spinach.