1.Preheat the oven to 220°C (200°C fan-forced). Tie the beef with kitchen string at 2cm intervals. Place the beef in a large roasting dish, brush with oil and sprinkle with salt.
2.Roast beef for 15 minutes. Reduce the heat to 180°C (160°C fan-forced). Roast, uncovered, for about 1 hour for medium rare or until the beef is cooked as desired. For accuracy, use a meat thermometer: rare â€“ 60°C; medium – 65°C-70°C; well done – 75°C. Remove the beef from the oven; cover with foil to keep warm. Stand for 20 to 30 minutes.
3.To make mustard bearnaise, place shallot, tarragon, peppercorns and vinegar in a small saucepan. Bring to the boil, simmer, uncovered, until reduced by half. Strain into a medium heatproof bowl.
4.Bring a medium saucepan half-filled with water to a gentle simmer. Place the bowl over the saucepan (ensuring that the base of the bowl does not touch the water); add egg yolks. Whisk vigorously until egg yolks are frothy. To avoid scrambling the eggs, remove the bowl from the heat periodically if the bowl gets too hot.
5.Slowly whisk in the melted butter, leaving behind white milk solids that are settled at the bottom. Whisk vigorously while incorporating.
6.Stir in extra tarragon and mustard, season well with salt and pepper. Cover tightly and keep warm until serving.
Don’t try to reheat the bearnaise as it will curdle. If the bearnaise is too thick, whisk in a few tablespoons of boiling water. If it does curdle, remove it from the heat. Whisk an egg yolk in a clean bowl then gradually whisk the curdled mixture into the egg yolk, whisking constantly.