In a small bowl whisk together the mustard, garlic, rosemary, olive oil, salt and pepper.
Trim off any excess fat that may lie beneath the fell, but not all, you might want to leave some to enrich the flavor of the meat and/or gravy. Make incisions with a knife that are about 1 inch long and 1/2 inch deep.
Apply the rub to the leg of lamb evenly, making sure to fill the incisions with some of the rub. Let the leg of lamb sit at room temperature for about half hour.
Preheat oven to 425 F degrees. You'll need a large roasting pan fitted with a rack, pour the broth in the bottom of the pan and transfer leg to the pan. Roast it for about 30 minutes, then reduce temperature to 325 F degrees and continue roasting until cooked to your liking. For medium rare, you want the temperature to be 135 F degrees, for more well done, you need it to be 160 F degrees, this will take a couple hours.
Let the lamb rest for 20 minutes before carving it.
While the lamb is resting, you can make the gravy by removing the rack from the roasting pan and placing the pan across 2 burners over medium-high heat. Add wine to the drippings and bring to a simmer while whisking a bit. In a small bowl whisk together the flour with the broth then pour it into the roasting pan and whisk until gravy thickens, should take about a minute. Pour into a gravy bowl and serve with lamb.