Preheat your oven to 325°F (163°C) to ensure even cooking.
Using a sharp knife, make small slits all over the leg of lamb, about 1 inch apart. Insert the slivers of garlic and small pieces of rosemary into each slit.
Generously season the leg of lamb with kosher salt and freshly ground black pepper, rubbing it into the meat for maximum flavor.
Place the leg of lamb in a roasting pan, bone side down. Roast for about 1 hour and 30 minutes, or until the internal temperature reaches 135°F (57°C) for medium-rare.
In the last 30 minutes of cooking, brush the leg of lamb with orange marmalade to create a beautiful glaze.
Once cooked, remove the lamb from the oven and let it rest for at least 15-20 minutes.
Carve the lamb into slices and serve with your favorite sides, drizzling some pan juices over the top.
