Special equipment: a mandoline
In a bowl, combine the onions and enough cold water to cover, and let them soak for 10 minutes. In a large heavy-bottomed pot, pour enough vegetable oil to fill the pan about a third of the way. Heat over medium heat until a deep-frying thermometer inserted in the oil reaches 365 degrees F.
Prepare a tray lined with paper towels. Put the cornstarch in a shallow bowl or pie plate.
Remove the onions from the water, drain and pat them dry with a kitchen towel. Dredge the onions lightly in the cornstarch, shake off the excess cornstarch, and in batches, add them to the hot oil. Move the onions around while frying to prevent them from clumping.
Transfer the onions from the oil to the prepared tray when they are golden, just before they turn brown, as they will continue to color after their removal. Season the onions with the five-spice powder, and salt, to taste.