Mix zest in a small bowl with 4 tablespoons finely chopped rosemary, 2 tablespoons minced garlic, 1 tablespoon lemon juice, 2 tablespoons olive oil, salt and pepper.
Place leg of lamb in roasting pan. Cut slits all over lamb and rub rosemary mixture over lamb, rubbing into slits.
Place leg of lamb into oven for 20 minutes.
Meanwhile, cut potatoes in half.
Mix 2 tablespoons chopped rosemary, 1 tablespoon minced garlic, 1 tablespoon oil and 1/2 teaspoon salt. Sprinkle over potatoes and mix.
Turn heat down to 350°F. Remove lamb from oven and place potatoes around lamb. Put back in oven and roast 40 – 50 minutes, stirring potatoes occasionally, until meat thermometer registers 140°F
Remove from oven and let rest for 15 minutes.
Transfer lamb to a cutting board and slice thin across the grain. Place on serving platter, surround with potatoes and garnish with parsley or a few rosemary sprigs.