Peel and cut the onions into 1/4-inch thick slices; separate into rings. In shallow bowl, stir together milk or buttermilk and ½ teaspoon of the salt; add onion rings and soak for 30 minutes, stirring occasionally.
In a bag, combine flour, remaining salt and pepper. Add onion rings in batches and shake to coat.
In deep-fryer or large saucepan, heat 2 inches oil to 375 degrees. Deep-fry rings, in batches without crowding for about 3 minutes or until golden brown.
Drain and keep warm on paper towel-lined baking dish in 250 degree oven