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.
Boneless Leg of Lamb: For 4-7 lb Boneless Leg of Lamb, roast in a 325°F oven roasting times as follows:
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
For Bone in Leg of Lamb: For this recipe I use a 5.5 pound leg of lamb that feeds about 6 to 8 people. Oven temperature should be 300°F.
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.
2. Feel free to eat the roasted vegetables when serving, but I find they can be a little over cooked myself and serve the lamb with mashed potatoes.