Whisk together evaporated milk, salt, oil and egg white. Add flour mixture and mix to combine. Adjust consistency with water to make a very thick batter.
Heat oil in a deep fryer or deep pan to 355°F, about 3-inches deep. Dip 1/4 of onion rings in batter, taking care to coat thoroughly. Carefully lower into hot oil and cook until dark golden brown, about 2 - 3 minutes. Remove and let drain on paper towels. Repeat with remaining onion rings.