Peel the onions and cut them into half-inch slices, then separate them into rings before refrigerating them in a container of cold water for half an hour.
After cooling, drain the onion rings and pat them dry if needed; the batter won't stick to wet rings.
Mix the flour, salt, eggs, milk, and oil. Beat until smooth.
Batter the onion rings until they are thickly coated, then deep fry in 375°F oil until golden brown on both sides, approximately 4-5 minutes.
Drain the onion rings on paper towels.