Trim fat from roast. If necessary, cut roast to fit into a 31/2- to 6-quart crockery cooker. In a small bowl combine oregano, lemon peel, garlic, and salt. Sprinkle evenly over sides of lamb roast; rub lightly with fingers. Place lamb in cooker. Sprinkle lamb with lemon juice.
Cover and cook on low-heat setting for 8 to 10 hours or on high-heat setting for 4 to 5 hours.
Remove lamb from cooker. Remove meat from bones; discard bones and fat. Chop meat; set aside. Add spinach to cooking juices in cooker, stirring until spinach is wilted. Add cooked orzo, feta, and lamb; stir to mix.