Remove the beef from the fridge 30 minutes before cooking. Preheat the oven to 190°C. Line a large shallow baking tray with foil or baking paper.
Using a sharp pointed knife, cut about 8 - 10 deep slashes through the fat layer of the beef to just penetrate to the meat beneath. Mix the mustard and tarragon together with a good seasoning of black pepper and spread over the beef.
Roast in the preheated oven for 1 hour or until the meat is cooked to your liking - this will cook the meat to medium/medium-rare depending on the thickness of the sirloin. To ascertain the degree of cooking, pierce the meat with a cook’s fork or skewer. If the juices are pink, the meat is cooked. If they are red, return to the oven for a few more minutes. Once cooked, allow to stand for 15 minutes to ensure the juices are absorbed into the meat, ensuring it’s tender to carve and eat. Serve thinly, sliced with the tarragon cream and your favourite vegetetables.
For the tarragon cream
Beat eggs, sugar and vinegar in a heatproof bowl. Place over a saucepan of simmering water and stir constantly until the mixture is thick. Remove and cool. Fold in the whipped cream and tarragon.