1. Roughly chop strips of lemon zest. In a medium bowl, combine lemon zest and juice, garlic, and rosemary; whisk in oil. Pat lamb dry and season with salt and pepper. Place lamb in a 1-gallon resealable bag and pour in marinade. Seal bag, pressing to remove as much air as possible. Turn bag several times to coat lamb in marinade. Let sit at room temperature 1 hour (or refrigerate overnight), flipping bag halfway through.

2. Preheat oven to 450 degrees. (If lamb was marinated overnight, remove from refrigerator 1 hour prior to cooking.) Place potatoes in a large pot and cover with cold water by 2 inches; season with salt. Bring to a boil; reduce to a simmer and cook until potatoes are tender when pierced with a knife, about 15 minutes. Drain potatoes.

3. Remove lamb from marinade, letting excess marinade drip back into bag. Place lamb on a plate; season generously with salt and pepper. Pour marinade into a roasting pan; add potatoes. Cook 15 minutes. Push potatoes to edges of pan; place lamb in center. Roast 15 minutes, then reduce oven temperature to 300 and cook until medium-rare (an instant-read thermometer should read 125 when inserted in thickest part of lamb), 1 to 1 1/4 hours, tossing potatoes halfway through.

4. Tent pan loosely with foil and let lamb rest 20 minutes before slicing. Toss potatoes with pan juices and serve alongside lamb.