Rinse lamb and pat dry; lay shanks side by side in a 12- by 17-inch pan about 2 inches deep. Bake in a 450° regular or convection oven, turning once, until meat is well browned all over, about 25 minutes total. Reduce oven temperature to 325°.
Meanwhile, place capers and olives in a fine strainer and rinse with cool water; drain. Mince rosemary or combine with about 1 cup of the wine in a blender and whirl until minced. Scatter capers, olives, and rosemary over lamb (or pour rosemary-wine mixture evenly over lamb) and add the wine (the rest of the wine if whirling some with rosemary); stir around shanks to scrape up browned bits. Sprinkle pepper and lemon peel over meat; add lemon juice to pan. Cover pan tightly with lid or foil.
Bake until meat is very tender when pierced and pulls easily from the bone, 3 to 3 1/4 hours.
Spoon equal portions of Lemon Couscous into wide, shallow bowls. With tongs, lift lamb shanks from pan and set one on couscous in each bowl. Skim and discard fat from juices in pan. Ladle juices with olives and capers over meat. Garnish each bowl with about 1/2 cup watercress sprigs.