Peel the onions leaving them whole and cut across into thin slices. Separate into rings and chill in iced water for about 20 minutes so that they are nice and crisp.
Drain the onion rings and pat dry. Beat the egg white with a fork, just enough to break up the white but not to make it frothy. Spear the onion rings on a fork and dip first in the egg white and then in seasoned flour. Shake away any excess coating and deep-fry the rings in hot fat for 1–2 minutes or until crisp and pale golden. Drain and serve at once.