Take the lamb out of the refrigerator and allow it to sit at room temperature for at least one hour.
Preheat the oven to a high temperature and position the rack in the middle.
Mix rosemary, garlic, and olive oil to form the rub.
Place the lamb in a roasting pan and coat it evenly with the rub. Sprinkle with salt and pepper. Arrange the halved garlic and rosemary sprigs underneath.
Roast the lamb at high heat first, then reduce the temperature and continue roasting until it reaches your desired doneness.
Remove the lamb from the oven and transfer it to a plate. Cover loosely with foil and let it rest before slicing.
For the gravy, place the roasting pan on the stove over medium heat. Add flour to the drippings and cook briefly.
Pour in the broth gradually, stirring to combine and dissolve the drippings.
Mash the roasted garlic into the gravy, then simmer until it thickens slightly. Strain if desired and serve alongside the lamb.