Combine half cup salad dressing,flour,salt;gradually add milk and beaten egg yolks.Cook,stirring constantly,over low heat until thickened.Remove from heat;cool 15 minutes.Fold in stiffly beaten egg whites.Pour into 15 x 10 x 3 cm jelly roll pan lined with greased wax paper.Bake @ 220ºC or 20 minutes.Invert onto towel. Carefully remove wax paper.Cover with combined remaining salad dressing,sausage, 0.75 cup cheese,and apple.Roll up omelet,starting at narrow end.Place,seam side down,on serving platter.Top with remaining cheese.Serves 6 to 8.