You need little more than a good sear and a roaring-hot oven to bring out the best in a top sirloin roast. Top sirloin has natural tenderness, and the only thing that toughens it is overcooking – anything over medium doneness results in a chewy bite. For maximum juiciness, let the sirloin rest for several minutes after roasting.
Prep the Sirloin
Heat the oven to 425 F. Liberally season the sirloin with kosher salt and freshly ground black pepper, and coat it with one or two tablespoons of oil. Tie the roast crosswise with kitchen twine at one-inch increments, just snugly enough to hold a uniform shape.
Sear the Sirloin
Heat one or two tablespoons of oil in an oven-safe skillet on the stove over medium-high heat for three to four minutes. Lay the roast in the pan and sear it until a rich, golden-brown crust forms, about three minutes per side. You'll need sturdy tongs to turn the roast as it sears to get a uniform crust.
Roast the Sirloin
Transfer the roast from the stovetop to the oven. If you didn't sear the roast in an oven-safe skillet, transfer it to a baking dish. Roast the sirloin for 40 minutes, then check its internal temperature in the center. Depending on the size of the roast, after 40 minutes, it should measure between 120 and 130 F, or medium rare. For a medium finish (135-145 F), roast the sirloin an additional four to five minutes, and for a medium-well finish (145-155 F) roast the sirloin for another six to seven minutes.
Rest the Sirloin
Take the pan from the oven, and transfer the sirloin to a carving board. Tent a piece of aluminum foil over the roast and set it aside for five to 10 minutes. While the roast rests, pour a couple tablespoons of water or stock in the pan and scrape up the fond (the stuck-on pieces of caramelized meat and reduced juices) using a wooden spoon or spatula.
Slice and Serve
Snip off the twine from the roast and slice it crosswise into 1/4-inch slices. Transfer the sirloin to a serving dish, and spoon the pan drippings over the slices.