Take out the lamb from the fridge at least 1 hour before roasting. Room temperature lamb cooks more evenly.
Preheat the oven to 350°F.
Place the lamb leg on a rack in roasting pan, fat side up.
Zest the lemon and juice it. You should get about 3 tablespoons of lemon juice.
In a small bowl, mix together lemon juice, zest, mint leaves, salt and pepper.
Spread the mixture all over the lamb.
Cut garlic head horizontally and place it in a roasting pan along with pearl onions. Add 1 cup of water, or beef broth.
Roast until desired doneness, 2-2.5 hours for medium doneness (135-140°F internal temperature). (See Note 2 below for additional information on doneness/internal temperatures.)
Remove from oven, cover with foil and rest for at least 15 minutes (or up to 1 hour) before slicing and serving.
To make red wine gravy, remove roasted garlic and pearl onions onto a plate. Pour the pan drippings (about ¼ cup) into a medium saucepan and heat it over medium heat. Once bubbling, stir in flour and toast, stirring continuously, until golden brown and smells nutty, 2-3 minutes.
Slowly pour in wine, while whisking, and then add beef broth. Squeeze in the roasted garlic. Cook until nice and thick, about 3 minutes.
Refrigerate leftover lamb in an airtight container for up to 3 days, or freeze for up to 3 months.