Remove beef from fridge and let it to come to room temp for half an hour.
Preheat the oven to 170c
Peel and quarter onions and arrange them on a roasting tray. tuck in some herbs, saving some to add on top of the beef. This forms a bed to roast the beef on and creates a nice gravy, I also add carrots and parsnips. That is optional
Season beef well with salt, then heat a large frying pan with some olive oil. sear the beef joint well all over, for about 3 minutes on each side, making sure to sear the ends.
place on to the bed of onions and rub in the whole grain mustard over the beef, season again with some salt and add some ground black pepper then add some of the left over herbs.
Roast in the oven for 1 hour. remove from oven and let rest for 15/20 minutes before slicing. Add any pan liquid into the gravy for more flavour
COOK THE MEAT FOR 20 MINUTES PER 500G (1/2 KG) PLUS 10 MINUTES