Peel and slice whole onions 1/8 to ¼-inch thick. Separate the rings and soak them in a mixture of half buttermilk and half whole milk for at least one hour.
In a deep fryer or heavy-bottomed pot, heat vegetable oil to 365°F.
Drain the onions and toss then with flour and shake in a colander or large strainer to remove excess flour.
Fry in small batches until golden brown. Drain on paper towels, salt and serve.