Remove the beef from the fridge and place it on a rack. Leave at room temperature for 30 minutes.
Preheat the oven to 200°C.
In a small bowl, mix all the glaze ingredients until well combined.
Brush the glaze generously all over the beef joint.
Arrange the lime slices in a roasting tin and place the beef on top. Add the onion halves around the meat.
Roast in the oven for 20 minutes, then reduce the temperature to 175°C.
Turn the beef over and continue roasting for another 20–30 minutes, depending on how well done you prefer it.
Transfer the beef to a plate, cover loosely with foil and rest while you make a gravy or jus from the roasting juices.
Carve the beef and serve with the pan juices and a scattering of chopped herbs.