Step 1: Begin by removing your boneless leg of lamb from the refrigerator at least 1-2 hours before cooking. This allows the meat to come closer to room temperature, promoting more even cooking. Pat it thoroughly dry with paper towels.
Step 2: If your lamb is butterflied and untied, now is the time to prepare it. Lay it flat, fat-side down. In a small bowl, combine your minced fresh rosemary, minced garlic, a generous amount of salt, black pepper, and a tablespoon or two of olive oil to create a thick paste.
Step 3: Spread this aromatic paste generously over the inside surface of the lamb. Roll the lamb tightly from one end to the other, ensuring the seasoned side is mostly enclosed. Secure the rolled lamb with butcher's twine at 1-inch intervals. This ensures it holds its shape and cooks uniformly. If your lamb came pre-rolled and netted, simply rub the herb mixture generously all over the exterior.
Step 4: Once tied, place the lamb on a rack in a roasting pan. Drizzle the exterior with a little more olive oil and sprinkle with additional salt and pepper.
Step 5: Preheat your oven to 425°F (220°C).
Step 6: Place the roasting pan with the lamb into the preheated oven and roast for 15-20 minutes at 425°F (220°C).
Step 7: After the initial high-heat roast, reduce the oven temperature to 350°F (175°C). Continue roasting, basting occasionally with any pan drippings, until the internal temperature reaches your desired doneness.
Step 8: For medium-rare, aim for an internal temperature of 130-135°F (54-57°C). For medium, target 135-140°F (57-60°C). Use a reliable meat thermometer inserted into the thickest part of the roast. Remember, the temperature will rise by 5-10 degrees during resting.
Step 9: Once the lamb reaches your desired temperature, remove it from the oven and transfer it to a cutting board. Tent it loosely with aluminum foil and let it rest for at least 15-20 minutes, or even up to 30 minutes for larger roasts. This resting period is NON-NEGOTIABLE!
Step 10: Carefully remove the butcher's twine. Slice the lamb against the grain into ½-inch thick slices. Serve immediately with your favorite side dishes.
Step 11: Don't forget the pan drippings! These can be deglazed with a splash of wine or broth to create a simple, flavorful au jus to serve alongside your beautiful boneless leg of lamb.