Cook sausage in nonstick large skillet over medium heat until fat is rendered, 5 to 6 minutes. Discard fat. Transfer sausage to large bowl. Melt butter in same skillet over low heat. Stir in onion, cover, and cook until translucent, about 10 minutes. Remove from heat. Add fresh herbs and sage to onion and toss gently to blend. Add breadcrumbs, Sherry, egg, lemon peel, salt, pepper and onion mixture to sausage and blend well; mixture should hold together. Let cool.
Preheat oven to 350F. Spread stuffing evenly over boned lamb. Roll into log shape, fat side out, and tie with string at 2-inch intervals. Rub entire surface with cut side of lemon. Sprinkle with evenly with Seasoning Flour, using enough to cover entire surface. Transfer lamb to parchment paper-lined roasting pan. Roast uncovered 1 hour.
Meanwhile, Prepare crust:
Mix all ingredients in small bowl, and set aside.
Remove lamb from oven and coat entire surface with crust mixture. Continue roasting until crust is golden and lamb is tender, pink and juicy, about 1 hour. Transfer to heated platter. Let stand 10 to 15 minutes before cutting into thick slices. Garnish each serving with watercress sprigs. Deglaze pan juices with additional Sherry and stock if desired. Reduce juices to saucelike consistency and serve.
Seasoning Flour: Combine all ingredients in small cup.
across surface of meat, spacing cuts about 1-inch apart. Rub with lemon, then sprinkle with Seasoning Flour. Cut pocket large enough to contain stuffing under fatty surface. Continue as described in recipe instructions.