A sirloin roast can't compare to a standing rib roast in terms of fat marbling, but it also doesn't come with prime rib's extravagant price tag. When treated with a savory herb marinade, a sirloin roast can rival traditionally prepared herb-roasted prime rib for flavor at a fraction of the cost. With a little patience, a sirloin roast can also approach the prime rib's decadent texture. Serve the roast in thick slices au jus to complete the illusion.
Dry the sirloin roast thoroughly with paper towels. Season it on all sides liberally with kosher salt and freshly ground black pepper.
Grind one or two bay leaves and black pepper, to taste, together in a spice grinder. Transfer the mixture to a mortar when it becomes powdery.
Add two or three cloves of garlic to the powder in the mortar. Add fresh herbs, such as rosemary, thyme, oregano and marjoram, according to taste. Grind the mixture with a pestle until it becomes a paste.
Slowly stir olive oil into the herb paste until it becomes the consistency of a thick marinade. Apply the marinade liberally to all sides of the sirloin.
Place the beef in a roasting pan fitted with a rack. Cover with a lid and refrigerate for 8 to 24 hours. The longer the sirloin is allowed to marinate, the closer it will come to the tenderness of prime rib.
Remove the roast from the refrigerator at least one hour before cooking. Preheat your oven to 450 degrees Fahrenheit. Place the sirloin, uncovered in its rack, in the center of the oven.
Roast the sirloin for 20 minutes at 450 F. Reduce the heat to 300 F and continue to roast until an instant-read thermometer inserted into the center of the roast registers 110 F, about 2 hours for medium-rare doneness.
Remove the roast from the oven and transfer to a large platter. Cover the beef loosely with aluminum foil and allow it to rest until an instant-read thermometer inserted into the center of the roast registers 130 F, between 20 and 30 minutes.
While the roast rests, remove the rack from the roasting pan and set the pan directly on a burner over medium-high heat. Add beef stock, garlic and herbs as desired.
To make the jus, bring the stock to a boil and scrape up any bits from the bottom of the pan with a wooden spoon. Allow the stock to boil for 1 to 2 minutes, until the pan drippings are mixed completely into the broth.
Reduce the heat to medium-low and add any juices that have accumulated on the beef's resting platter. Allow the jus to simmer gently until very aromatic, about 10 minutes.
Remove the butcher's twine. Slice beef in portions about 3/4-inch thick and serve with the sauce, au jus.