Peel, wash, and cut the onions crosswise into 1/4" slices.
Separate the rings and soak in milk for 1 hour.
Sift the flour and salt together.
Add the egg yolks and the 2/3 cup of milk.
Beat in the butter.
Add the egg white beaten stiff.
Dip the rings in the batter and fry in deep fat at 365° F. until the onions are lightly browned.
Drain on absorbent paper.