Tender, boneless leg of lamb is just the thing for a cold winter’s day. Roasted with vegetables, herbs, and seasonings, it becomes impossibly moist, juicy, and flavorful. Serve with mashed potatoes for a world class dinner!
Cooking with lamb seems daunting, but it certainly doesn’t have to be. My slow roast leg of lamb recipe is fool-proof. With simple seasonings and ingredients, the hardest part is having to smell this delicious roast while it slowly cooks in the oven!
When I make this dish, I use a 5.5 lb bone-in leg of lamb. However, I have included instructions for both bone-in and boneless leg of lamb so that you can prepare this meal based on your favorite cut of meat — or whatever you are able to find in the grocery store. It is delicious either way!
INGREDIENT NOTES AND SUBSTITUTIONS
- Lamb – A protein with a strong, earthy flavor. Your cook time will vary depending on choice of cut and weight.
- Onion, Celery, and Carrots – When roasted with these vegetables, the meat soaks up all of their mildly sweet, earthy flavors. The aroma of all three cooking together is unmatched! Yellow or white onions will both work well in this dish.
- Orange – This fruit also contributes to the dish’s tantalizing aroma, helping to tenderize and add a degree of sweetness.
- Garlic – Have 15 to 20 cloves on standby for this dish. Due to its role in the pan gravy, I would not substitute this ingredient.
- Rosemary – Pairing beautifully with the meat, carrots, and orange, this herb has a complex flavor profile that matches well with citrus, sweet, and peppery notes. Thyme or sage could be used, if necessary.
- Beef Broth – We will roast the meat in a mixture of beef broth and water, helping it to moisten and tenderize.
- Flour – Use this to thicken the leftover juice from the roast, creating a mouthwatering pan gravy to serve with our dish.
HOW TO MAKE BONELESS LEG OF LAMB
- Preheat. Preheat your oven based on the baking instructions for your chosen cut of meat below.
- Slice the Vegetables. Cut the onion and orange into quarters. Separate and remove the skins of your garlic. Slice the celery into uneven, large chunks. Add to a pan alongside the carrots and rosemary.
- Season the Meat. Sprinkle salt and pepper over the entire surface, pressing and massaging the seasonings into the meat. If using bone-in, lay the cut down with the bone side up. Pour the beef broth and water into the roasting pan. Finally, pour the olive oil over the meat. Cover the pan with a large piece of aluminum foil.
HOW TO ROAST BONE-IN VS. BONELESS LEG OF LAMB
Boneless: Cook based on the weight of the meat. For a 4-7 lb cut, set your oven to 325°F. Adjust roasting times based on desired doneness:
- Medium Rare: Cook for 20 minutes per pound, with an internal temperature of 145°F.
- Medium: Cook for 25 minutes per pound, with an internal temperature of 160°F.
- Well-Done: Cook for 30 minutes per pound, with an internal temperature of 170°F.
Bone-In: Again, adjust based on how many pounds of meat you’re cooking. Set your oven to 300°F.
For a 4 to 4.5 lb cut, cover the pan and cook for 4.5 hours. Remove the cover and roast for 45 minutes longer to achieve a golden brown crust. If the meat is 6 to 6.5 lbs, roast covered for an additional hour.
HOW TO MAKE PAN GRAVY FOR SLOW ROAST LEG OF LAMB
- Remove the Veggies. Transfer the pan, with all the juice and vegetables still inside, to your stovetop. Remove the vegetables, rosemary, and orange.
- Mash and Boil. Using a masher or wooden spoon, mash the roasted garlic. Mix this with the juices in the pan, then heat the juice until it begins to boil.
- Thicken with Flour. In small increments, whisk in the flour until everything is well combined. Then, simmer the gravy over low heat until it thickens.
Note: If your gravy is too thick, add small amounts of water until you have reached your desired consistency. Alternatively, mix in additional flour until it is as thick as you like. If desired, strain the bits of herbs remaining in the mixture.
Is bone-in or boneless leg of lamb better?
There are plenty of pros and cons for either. Bone-in can be more flavorful but trickier to slice and serve. It also takes longer to cook. Boneless does have a quicker roasting time and is easy to carve, but this cut might be more expensive.
How long does it take to slow roast leg of lamb?
It takes about 5 hours and 45 minutes to slow roast a boneless leg of lamb.
What can I serve with roasted lamb?
I like to serve this dish with mashed potatoes! Since the vegetables used for roasting will be mushy and overcooked, I recommend roasting a separate batch alone if you wish to enjoy them as a side.
Video: How to oven roast boneless leg of lamb
Want to see the recipe in action from start to finish? Just watch the video below!
Bone-In or Boneless Leg of Lamb
- 5 1/2 lb leg of lamb (bone in) (For boneless, See Note 1)
- black pepper
- 1 large onion quartered
- 1 large orange quartered
- 2 ribs celery
- 2 cups cut carrots
- 1 large head of garlic (about 15-20 cloves)
- 3-4 fresh rosemary sprigs (6+ inches long)
- 4 cups beef broth
- 2 cups water
- 2 tbsp olive oil
- 1/4 cup all purpose flour
- Preheat oven to 300°F.
- Cut and quarter the onion and orange. Break up the head of garlic and remove skins or have 15-20 cloves peeled and ready. Rough chop the celery and set all in the bottom of a large roasting pan with the carrots and rosemary.
- Lay the leg of lamb on top and season generously with kosher salt and black pepper on both sides. Leave the bone side up and pour in the beef broth and water. Drizzle the olive oil on top of lamb and then cover with aluminum foil and place in the hot oven for 5 hours (See Note 2 for different weights and roasting times).
- Remove from oven and set aluminum foil aside. Carefully flip leg of lamb, baste with pan juices and return to oven to roast, uncovered for 45 minutes or until the outside is a rich, golden brown.
- Remove lamb from pan and keep warm on a serving plate with the aluminum foil covering.
For Pan Gravy
- Move roasting pan to stove top over 2 burners. Using a slotted spoon remove the vegetables (See Note 3), rosemary sprigs and citrus, leaving the roasted garlic. Mash the garlic with the back of a wooden spoon and bring the pan juices to a boil. Sprinkle the flour over the gravy and whisk to incorporate. Turn heat to low and simmer until thickened. Add water if too thick, or cook longer to reduce if gravy is thin. Feel free to strain if you do not want the garlic/rosemary pieces and serve alongside roasted leg of lamb. I serve with mashed potatoes.
- Medium Rare (internal temp should be 145°F) 20 minutes per pound
- Medium (internal temp should be 160°F) 25 minutes per pound
- Well Done (internal temp should be 170°F) 30 minutes per pound
- 4 to 4.5 pound leg of lamb roast covered for 4.5 hours and uncovered for another 45 minutes or until the outside is a rich, golden brown.
- 6 to 6.5 pounds, roast covered for 5.5 hours and uncovered for another 45 minutes or until the outside is a rich, golden brown.
The information shown is an estimate provided by an online nutrition calculator. It should not be considered a substitute for a professional nutritionist’s advice.