Combine 1/2 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 1" jelly roll pan lined with greased wax paper.Bake @ 425 degrees for 20 minutes.Invert onto towel. Carefully remove wax paper.Cover with combined remaining salad dressing,sausage,3/4 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.