Costco Boneless Leg Of Lamb Recipe: Garlic-Rosemary Slow Roasted

This slow-roasted leg of lamb transforms a Costco cut into a show-stopping centerpiece with minimal effort. The garlic and rosemary create a fragrant crust that locks in moisture.

Cooking it low and slow ensures tender, fall-apart meat every time. The result is a dish that’s impressive yet approachable for home cooks.

Ingredients for Costco Boneless Leg of Lamb

Transform a Costco cut into an elegant centerpiece with these fragrant, simple ingredients.

The lamb is the foundation, and Costco’s boneless leg is ideal—consistently high quality, well-trimmed, and priced right. Select a cut with even marbling for optimal tenderness. Fresh rosemary and garlic are non-negotiable here; their aromatics define the crust. Look for rosemary with vibrant green needles and a strong, piney scent. For the olive oil, opt for a robust, extra-virgin variety to complement the richness of the meat.

No white wine? Dry vermouth or additional chicken stock works in a pinch. If rosemary is hard to find, fresh thyme at a 1:1 ratio offers a similar earthy depth. For a sweeter glaze, swap honey for maple syrup. The Dijon mustard adds tang and helps the crust adhere—whole grain mustard can be used but will change the texture slightly. Carrots and onions can be swapped for other root vegetables like parsnips or celery root.

This dish reheats beautifully, making it perfect for leftovers. Store cooked lamb in an airtight container in the fridge for up to 4 days. For longer storage, freeze sliced portions for up to 3 months. The flavors deepen over time, so it often tastes even better the next day.

Step-by-Step Instructions for Garlic-Rosemary Slow Roasted Boneless Leg of Lamb

Master the technique for tender, aromatic lamb with a golden crust and rich pan juices.

Step 1: Prepare the Lamb with Garlic and Herb Rub

Pat the 4 lb Boneless Leg of Lamb completely dry with paper towels to ensure optimal browning. Using a sharp knife, make small incisions all over the surface and insert the 8 cloves Garlic Cloves slivers into these cuts.

In a small bowl, combine the 6 sprigs Fresh Rosemary Sprigs (finely chopped), 1.5 tbsp Kosher Salt, 1 tbsp Freshly Ground Black Pepper, and 2 tbsp Dijon Mustard. Rub this mixture evenly over the entire surface of the lamb, pressing to adhere. Drizzle with 0.25 cup Extra-Virgin Olive Oil, massaging it into the meat. Let it sit at room temperature for 30 minutes to absorb the flavors.

Step 2: Sear the Lamb for Deep Flavor Development

Preheat your oven to 325°F. Heat a large oven-safe Dutch oven or roasting pan over medium-high heat. Add the lamb and sear on all sides for 2-3 minutes per side until deeply browned. This step builds a flavorful crust and enhances the final dish.

Remove the lamb and set it aside temporarily. In the same pan, add the 1 whole Yellow Onion (roughly chopped) and 3 whole Carrots (peeled and cut into chunks). Sauté for 3-4 minutes until they begin to caramelize, which will add depth to the pan sauce later.

Step 3: Slow Roast with Aromatic Liquids

Return the lamb to the pan, placing it on top of the vegetables. Pour in the 0.5 cup Dry White Wine and 0.5 cup Chicken Stock, ensuring the liquids pool around the base but do not submerge the lamb. This creates a moist environment for even cooking.

Cover the pan tightly with a lid or foil and transfer it to the preheated oven. Roast for 2 to 2.5 hours, basting the lamb with the pan juices every 30 minutes. The lamb is done when an instant-read thermometer inserted into the thickest part reads 145°F for medium-rare.

Step 4: Glaze and Rest for Optimal Tenderness

Zest the 1 whole Lemon and stir the zest along with 1 tbsp Honey into the pan juices. Increase the oven temperature to 400°F and return the lamb to the oven, uncovered, for an additional 10-15 minutes to develop a glossy, caramelized glaze.

Transfer the lamb to a cutting board, tent loosely with foil, and let it rest for 15-20 minutes. This allows the juices to redistribute, ensuring each slice is moist and flavorful. Skim excess fat from the pan juices and serve the sauce alongside the carved lamb.

Pro Cooking Tips for This Dish

Thermometer Precision

Invest in a instant-read thermometer—I use ThermoWorks Thermapen—for perfect results. Check the thickest part of the lamb, avoiding bone or fat. Pull it at 140°F for medium-rare; it’ll rise to 145°F while resting. This prevents overcooking and dry meat. 🌡️

Resting Matters

Let the lamb rest for 15-20 minutes after roasting. Tent it loosely with foil to keep it warm. Skipping this step means juices will spill onto the cutting board instead of staying in the meat where they belong.

Basting Technique

Baste the lamb every 30 minutes with pan juices to keep it moist and build flavor. Use a long-handled spoon to avoid opening the oven door too long. This ensures even browning and a glossy finish.

Pan Sauce Perfection

After removing the lamb, place the pan on the stovetop over medium heat. Add a splash of wine or stock to deglaze, scraping up browned bits. Simmer for 3-4 minutes until slightly reduced for a rich, flavorful sauce. 💫

Carving Tips

Slice against the grain into ½-inch thick pieces for maximum tenderness. Use a long, sharp knife—Wüsthof or Victorinox are my go-tos. Angle the knife slightly for cleaner cuts through the crust.

Oven Hot Spots

Rotate the pan 180 degrees halfway through cooking if your oven has hot spots. This prevents uneven browning. I learned this the hard way after one side came out darker than the other.

Frequently Asked Questions