Add ginger and water to a saucepan. Bring to a boil, then turn the heat down to medium-low, cover, and simmer until tender, about 35 minutes. The slices will be a deeper shade of yellow and have a more rubbery consistency. If your ginger is tougher, this step could take much longer. Keep adding water as necessary. When tender, drain and reserve ¼ cup cooking liquid. (Reserve the rest of the drained water if you wish to use it later.)

Return the ginger to the pot along with the 1/4 cup of cooking liquid and the sugar. Bring to a rapid simmer over a high flame. Turn the heat down to medium and continue to simmer the ginger, stirring frequently, until the liquid has evaporated and the sugar begins to crystallize, about 15 to 20  minutes.

Transfer the ginger immediately to a cooling rack or parchment-lined sheet pan. Spread in an even layer until cool enough to touch. Reserve excess syrup or sugar for another use.

Store the candied ginger in an airtight container for up to several weeks. See the accompanying post for more information.