TO MAKE SAUCE, place the beef stock in a large saucepan with the peppercorns. Roughly chop the onion and add it to the pan. Bring to the boil over a high heat, then reduce to a simmer. Let the stock simmer until it has reduced by half, skimming occasionally. Remove from the heat and strain through a fine sieve. Set aside.
TO PREPARE MEAT, slice the beef into medallions abo