1. Calculate roasting time at 200 ºC for 10 minutes, then at 170 ºC for 15 to 20 minutes for every 500 g, depending on thickness (55 to 80 minutes). Preheat oven to 200 ºC.
2. Make incisions in beef and insert slivers of garlic.
3. Just before roasting, blend oil and pepper and smooth over sirloin. Sprinkle mustard and salt all over meat.
4. Place meat on rack of roasting pan, fat side up, and roast, uncovered, for calculated time. Check internal temperature with an oven thermometer.
5. Prepare and roast potatoes, as described below.
6. Allow meat to stand for 15 minutes in a warming drawer, or in a warm place, to make it easier to carve.
OVEN ROASTED POTATOES
Peel and quarter potatoes. Parboil for 10 to 15 minutes, drain and remove from saucepan. Sprinkle with salt, milled black pepper, olive oil, crushed garlic, and origanum sprigs. Place in roasting pan directly under meat and roast with meat, turning once, for crisp roast potatoes. Continue browning for a little longer, after removing the meat, if necessary.