In a large saucepan bring stock to a boil over high heat. Taste for seasoning, add salt and pepper as needed, and stir in orzo. Bring to a boil, cover and reduce heat to simmering. Cook until orzo is tender, about 10 minutes. In a bowl whisk egg whites to soft peaks, then beat in yolks. Mixture should be frothy. Stir in lemon juice until just combined. Slowly stream 2 ladlefuls of hot stock over egg mixture, whisking constantly. Slowly whisk egg mixture back into simmering stock, stirring until thickened. Do not let soup boil. Serve immediately, garnished with parsley.;