Set the oven at 220°C/gas mark 8. Crush the peppercorns roughly with a pestle and mortar and mix them with the salt. Rub the beef with a little olive oil then roll in the seasoning, pressing down so that most of the salt and pepper sticks to the meat.
Warm 2 tbsp of olive oil in a roasting tin then put over the heat till the oil starts to sizzle. Brown the meat quickly on all sides then roast for 10 minutes only. Remove from the oven and leave to cool.
To make the dressing, put the mustard, lemon juice, mint leaves and egg yolks in a blender and whiz for a few seconds. Pour in the oil, slowly, stopping when you have a dressing the consistency of double cream.
Slice the beef very thinly, then spoon over the dressing at the table.