Step 1

Crush the peppercorns with a pestle and mortar then mix in the salt and mustard powder. Sprinkle onto a large plate. Brush the beef with some of the oil and roll in the seasoning.

Step 2

Heat remaining oil until very hot in a large heavy-based pan. Put the beef in the pan and turn heat to medium. Sear for 30 minutes, turning every 6-7 minutes until the seasonings form a deep brown crust. This will be rare in the middle. For well-cooked, add 10 minutes. Leave to cool on a plate. Boil potatoes for 10-15 minutes until tender. Drain, toss in the butter.