COMBINE flour and salt in small bowl. Cut onions into 1/4-inch slices. Separate into rings. Soak onion rings in milk; dip in flour mixture.
HEAT 2 to 3 inches (once melted) shortening in deep saucepan or deep fryer to 375ºF. Fry a few onion rings at a time until lightly browned, 2 to 3 minutes.