Pound garlic and 4 teaspoons coarse salt inmortar with pestle until paste forms.
Add 1/4cup rosemary leaves and lemon peel. Pound1 minute. Stir 1/4 cup oil, lemon juice, and 2teaspoons pepper into rosemary paste.
Place lamb, fat side up, on worksurface.
Cut shallow slits in fat about 1/2 inchapart.
Spread 5 tablespoons rosemary pasteover lamb, working into slits. Turn lambover; repeat, cutting slits and spreadingwith remaining paste. Wrap lamb in plastic.
Place in roasting pan; chill overnight.
Preheat oven to 425°F.
Remove lambfrom pan; brush pan with oil.
Removeplastic wrap from lamb; return to roastingpan, fat side up. Roast lamb 15 minutes.
Place potatoes in large bowl.
Add 1/4 cupoil and 2 tablespoons chopped rosemary.
Sprinkle with salt and pepper; toss to coat.
Reduce oven temperature to 350°F.Arrange potatoes, cut side down, inroasting pan around lamb. Roast 40minutes. Using spatula, carefully turnpotatoes over; brush with pan drippings.Roast until potatoes are tender, about 15minutes longer.
Transfer potatoes to platter.Continue to roast lamb until instant-readthermometer registers 130°F to 135°F wheninserted into thickest part, about 15 to 30minutes 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.
