Pound garlic and 4 teaspoons coarse salt in mortar with pestle until paste forms. Add 1/4 cup rosemary leaves and lemon peel. Pound 1 minute. Stir 1/4 cup oil, lemon juice, and 2 teaspoons pepper into rosemary paste.
Place lamb, fat side up, on work surface. Cut shallow slits in fat about 1/2 inch apart. Spread 5 tablespoons rosemary paste over lamb, working into slits. Turn lamb over; repeat, cutting slits and spreading with remaining paste. Wrap lamb in plastic. Place in roasting pan; chill overnight.
Preheat oven to 425°F Remove lamb from pan; brush pan with oil. Remove plastic wrap from lamb; return to roasting pan, fat side up. Roast lamb 15 minutes.
Place potatoes in large bowl. Add 1/4 cup oil and 2 tablespoons chopped rosemary. Sprinkle with salt and pepper; toss to coat.
Reduce oven temperature to 350°F Arrange potatoes, cut side down, in roasting pan around lamb. Roast 40 minutes. Using spatula, carefully turn potatoes over; brush with pan drippings. Roast until potatoes are tender, about 15 minutes longer. Transfer potatoes to platter. Continue to roast lamb until instant-read thermometer registers 130°F to 135°F when inserted into thickest part, about 15 to 30 minutes longer, depending on thickness. Transfer lamb to platter; let rest 20 minutes. While lamb rests, return potatoes to pan; warm in oven 10 minutes to heat through.
Arrange potatoes and lamb on platter.