Set oven Gas Mark 6, 200°C (400°F).
Place the beef on the roasting tin, crush the black peppercorns coarsely, and sprinkle over the fat of the beef, pressing them in quite well. Drizzle over the olive oil. Place the meat in a hot oven for 15-25 minutes per lb, depending on how you like the beef cooked. Reduce the oven to gas mark 4, 180°C (350°F) after the first 20 minutes.
Baste the joint well during cooking.
While the meat is cooking, start on the sauce. Put the vinegar, peppercorns, bay leaf and onion into a small saucepan. Boil until the mixture has reduced to about 3 tablesp. and set aside. Now whisk the egg yolks and mustard together in the top of a double saucepan of simmering water. Stir the vinegar mixture through a fine sieve on to the egg yolks. Add a tablesp. of water and beat over hot, but not boiling water, using a wire whisk. Add the butter, a knob at a time, still whisking after each addition until all the butter is in and the sauce has thickened. Add the herbs. Pick out 2 pimentos from the tin and chop in little pieces. Add to the sauce and season. Keep warm. The finished sauce should have the consistency of whipped cream.