Heat oven to 240C/220C fan/gas 9. Mix the flour and mustard powder with some seasoning, then rub all over the beef. Put the onion and carrots into a roasting tin and sit the beef on top, then cook for 20 mins.
Reduce oven to 190C/170C fan/gas 5 and continue to cook the beef for 30 mins if you like it rare, 40 mins for medium and 1 hr for well done.
Remove the beef and carrots from the oven, place onto warm plates or platters and cover with foil to keep warm. Let the beef rest for 30 mins while you turn up the oven to cook your Yorkshire puds and finish the potatoes.
For the gravy, put the tin with all the meat juices and onions back onto the hob. Stir in the flour, scraping all the stuck bits off the bottom of the tin. Cook for 30 secs, then slowly stir in the stock, little by little. Bubble to a nice gravy, season, then serve with the beef, carved into slices, carrots and all the other trimmings.