Lamb sirloin is the cut of meat located just in front of the lamb leg and is sometimes sold along with the leg. Often cut into steaks or chops, it can also be purchased as a boneless roast. A 3-ounce serving of roasted lamb sirloin is an excellent source of protein, vitamin B-12, niacin and zinc. But it is also high in fat, saturated fat and cholesterol. A diet that includes a large amount of red meats such as lamb may increase your risk of developing heart disease. When you're cooking a lamb sirloin roast, choose a low-fat preparation method like grilling.
Remove the lamb sirloin roast from its packaging and put it on a plate. Cover the roast with a clean dish cloth or aluminum foil and allow it to stand at room temperature for about 10 minutes.
Trim off and discard as much surface fat from the lamb sirloin as possible using a sharp knife. Butterfly the sirloin by making a lengthwise, horizontal cut nearly all the way through the meat, then opening the two equal halves of the roast like a book.
Rub the lamb sirloin with a mixture of olive oil and seasonings, such as lemon juice or zest, garlic, cumin, rosemary, salt and freshly ground pepper. Allow the coated lamb to sit for approximately 10 minutes.
Heat a gas or charcoal grill. Cook the lamb until a meat thermometer registers 130 degrees Fahrenheit, or about 8 minutes on each side, for an 8-ounce, boneless lamb sirloin roast.
Transfer the grilled lamb sirloin to a serving platter with metal tongs. Leave the meat to rest, undisturbed, for up to 15 minutes before serving.