Preheat oven to 350°.
Mix garlic, rosemary, salt and pepper together. Slowly add the olive oil to form a paste.
Rub paste into the lamb sirloin, pressingly down firmly to get the garlic to adhere.
Heat a cast- iron skillet over medium- high heat. When hot, add sirloin and brown on all sides- about 2-3 minutes per side.
Transfer skillet to oven and roast until the internal temperature reaches 130°, about 35- 45 minutes depending on size.
Remove from oven and let rest for 10-15 minutes. Slice and serve over a bed of carrot or root vegetable puree.