Peel the onions and cut them into one-quarter-inch slices. Separate the slices into rings.
Combine the flour, salt, evaporated milk, water and oil in a mixing bowl. Blend lightly but thoroughly. Fold in the egg white.
Heat the fat to 375°F. on a frying thermometer. Dip the onion rings into the batter. Fry them, a few at a time, turning once, for about one minute, until they are golden brown. Drain on paper towels and keep hot until all are finished.