Costco Boneless Leg of Lamb Recipe: Slow-Roasted Herb-Infused Tenderloin

This slow-roasted method transforms Costco’s boneless leg of lamb into a tender, herb-infused centerpiece with minimal effort.

By cooking low and slow, the meat becomes fall-apart tender while the herbs and garlic create a flavorful crust. Perfect for special occasions or Sunday dinners.

Ingredients for Costco Boneless Leg of Lamb

Costco’s boneless leg of lamb plus pantry herbs create this showstopping centerpiece with ease.

The lamb is non-negotiable—Costco’s boneless leg is consistently high quality with good marbling. For herbs, fresh rosemary and thyme are essential; dried won’t deliver the same aromatic punch. Look for sprigs with vibrant green leaves, not woody stems. Whole Foods or a well-stocked grocery store will have the best selection. The garlic should be fresh, firm heads with no sprouting—older cloves turn bitter when roasted.

No red wine? Substitute beef stock with 1 tbsp balsamic vinegar for similar depth. Fresh herbs can be swapped for dried at one-third the amount, though the flavor won’t be as bright. If you’re missing Dijon, whole-grain mustard works in a pinch. For the stock, Better Than Bouillon makes a great low-sodium beef base. The lemon zest adds brightness—don’t skip it (bottled juice won’t cut it).

The marinated lamb can sit overnight—actually improves flavor. Roast leftovers keep 4-5 days refrigerated or freeze for 3 months. Reheat gently in the oven with a splash of stock to prevent drying. Always bring lamb to room temperature before cooking for even roasting.

Step-by-Step Instructions for Slow-Roasted Herb-Infused Leg of Lamb

Master this technique for fall-off-the-bone tenderness with herbaceous depth in every bite.

Step 1: Prepare the Herb Marinade

In a bowl, combine the 0.5 cup Extra-Virgin Olive Oil, 3 tbsp Dijon Mustard, 1.5 tbsp Kosher Salt, and 1 tbsp Freshly Ground Black Pepper. Finely chop the 6 whole Fresh Rosemary Sprigs and strip the leaves from the 8 whole Fresh Thyme Sprigs, adding both to the mixture.

Mince the 8 cloves Garlic Cloves into a paste and incorporate it into the marinade. Zest the 1 whole Lemon and add the zest to the bowl, reserving the juice for later. Whisk everything until fully emulsified—the herbs should be evenly suspended in the oil.

Step 2: Coat and Marinate the Lamb

Pat the 5 lb Boneless Leg of Lamb dry with paper towels to ensure the marinade adheres properly. Place it in a large roasting pan or dish and pour the herb marinade over the lamb, using your hands to rub it into every crevice.

Let it marinate at room temperature for 1 hour, or refrigerate it for up to 12 hours for deeper flavor penetration. The lamb should develop a slightly tacky, fragrant coating as the herbs and oil work their magic.

Step 3: Sear and Begin Roasting

Preheat your oven to 325°F. Heat a large Dutch oven or oven-safe pot over medium-high heat. Remove the lamb from the marinade (reserve the marinade) and sear it on all sides until deeply browned, about 3-4 minutes per side.

Transfer the lamb to the Dutch oven and arrange the 4 whole Carrots and 1 whole Yellow Onion around it. Pour the 1 cup Red Wine and reserved marinade into the pot, then add the 1 cup Beef Stock. The liquid should come about halfway up the lamb.

Step 4: Slow-Roast to Tender Perfection

Cover the Dutch oven and place it in the preheated oven. Roast for 2.5 to 3 hours, turning the lamb halfway through the cooking time. The lamb is done when it reaches an internal temperature of 145°F for medium-rare or 160°F for medium, and a knife slides in effortlessly.

The vegetables should be caramelized and tender, and the pan juices will have reduced into a rich, aromatic liquid. If the liquid reduces too much during cooking, add a splash of water or additional beef stock to maintain moisture.

Step 5: Rest and Serve

Transfer the lamb to a cutting board and tent it loosely with foil. Let it rest for 15-20 minutes to allow the juices to redistribute. This step ensures maximum tenderness and flavor in every slice.

Meanwhile, skim any excess fat from the pan juices and stir in the reserved lemon juice for brightness. Slice the lamb against the grain and serve it with the roasted vegetables and pan sauce drizzled over the top. The meat should be impossibly tender, with the herb crust adding a fragrant finish to each bite.

Essential Equipment for Great costco boneless leg of lamb

Dutch Oven Essentials

A heavy 5-6 quart Dutch oven is non-negotiable for even heat distribution and moisture retention. I swear by my Lodge enameled cast iron—it handles the stovetop sear and oven roast seamlessly. The tight-fitting lid locks in flavors as the lamb slow-cooks. 🔥

Meat Thermometer

An instant-read digital thermometer is your best friend for perfect doneness. I use ThermoWorks ThermoPop for its accuracy and speed. Aim for 145°F in the thickest part for medium-rare. Never guess with lamb—it’s too precious.

Sharp Carving Knife

A long, sharp slicing knife (at least 8 inches) ensures clean cuts against the grain. My Wüsthof Granton Edge glides through without shredding the meat. Dull knives crush the fibers, ruining your tender roast.

Marinade Mixing Bowl

Use a large stainless steel bowl for the herb marinade—it won’t absorb odors like plastic. A 1.5 quart size gives plenty of room to whisk the olive oil and herbs. I keep one dedicated just for meat prep.

Roasting Rack Option

For extra browning, place the lamb on a wire rack inside your Dutch oven. Elevating it allows heat to circulate underneath. OXO’s stainless steel rack fits most pots perfectly. The carrots and onions can still catch the drippings below.

Tongs for Turning

Spring-loaded 12-inch tongs give you control when searing and flipping the lamb. I prefer OXO’s locking tongs—they grip securely without piercing the meat. A long handle keeps your hands safe from the heat.

Frequently Asked Questions