Meringue: Heat oven to 275 F. Beat egg whites until foamy, add cream of tartar and beat to stiff peaks. Gradually add 1 cup sugar and continue beating to very stiff peaks. Spread over a 9-inch pie plate which has been greased thoroughly, just to the edge. Sprinkle edge with finely chopped walnuts. Bake 1 hour. Cool. Filling: Beat egg yolks slightly and add 1/2 cup sugar, lemon juice, grated orange rind and salt. Cook over boiling water, stirring constantly until thick, about 10 minutes. Fold in 2 oranges which have been peeled and diced. Cool and fold in 1 cup whipped cream. Pour this mixture into center of meringue and smooth over. Chill 12 hours or longer in refrigerator. Just before serving, top with mounds of whipped cream (1 cup), leaving room for center to be filled with orange sections. Top cream with grated orange rind and decorate around center edge with orange slices.