Like most deep-fried foods, properly cooked onion rings get their appeal from the contrast of a crisp, savory exterior against the firm but yielding interior. For best results, it's crucial that the bread crumbs adhere to the onions, a result that's greatly enhanced by using the traditional trio of flour, egg wash and bread crumbs. You also must take care to maintain proper fryer temperature — too-hot oil yields rings that are overcooked outside and still raw inside, while too-cold oil will produce greasy, soggy results.
Peel the papery outer layer from the onions and slice them crosswise into 1/4-in. to 1/2-in. rings.
Arrange three shallow bowls, plates or similar containers in a row to set up your breading station.
Put flour in the first bowl and season with salt, pepper or spices as desired. The flour helps the egg wash adhere to the onion rings. In the second bowl, break eggs and whisk in a small amount of water to thin them out enough for easy dipping. Put the bread crumbs in the third bowl.
Dust an onion ring in the flour until well-coated. Dip it thoroughly in the egg wash, then transfer it to the bread-crumb bowl. Dredge or toss the onion ring in the bread crumbs until covered. Set aside on wax paper or a plate in a single layer and repeat with the remaining onion rings.
Heat the oil to 350 degrees Fahrenheit. You can check the temperature with a deep-frying thermometer or drop in a bit of breading. It should sizzle immediately and begin to turn golden within the first minute.
Add the onions to the fryer oil in batches, using a fryer basket or tongs. If you're using a skillet or wok, turn them once to brown properly on both sides. Be careful not to overload the fryer, which will lower the oil temperature and result in a greasy batch.
Drain the onions on paper towels and serve immediately.