1) Preheat the oil in a heavy pot over medium-high heat to 190C/Gas 5.
2) Slice the leeks into 1-cm wide rings, separating them into two layers at a time.
3) In a medium mixing bowl, whisk together the milk and egg. In another medium mixing bowl, combine the flour and salt. Divide the flour into two separate, shallow dishes and place the milk and egg mixture in a third.
4) Going one small handful at a time, dip the rings into the first flour mixture, then into the milk and egg, and then into the second flour mixture. Working in batches, fry the rings for 1 to 1 1/2 minutes, or until golden brown. Remove the rings to a cooling rack set inside a half sheet pan.
Allow the leek rings to drain for 2 to 3 minutes before serving. Season with additional salt, if desired.