Peel the ginger with a sharp teaspoon and slice it finely using a mandoline slicer or sharp knife. Add it with ½ tsp salt in a small bowl, mix, and set aside for 5 minutes.
Bring a small saucepan with water to the boil. Add salted ginger and cook for 1 minute (up to 5 minutes for less spicy and mild sushi ginger). Drain over a sieve, wait until cool, and squeeze out all excess liquid using your hands. Fill the ginger in a small mason jar.
Add rice vinegar, sugar, and salt into the same saucepan. Bring to a boil, switch off the heat, and stir until the sugar has dissolved. Pour it over the ginger and if necessary, press the ginger down with a tablespoon so that it is fully covered with the liquid. Close the lid, let it cool, and refrigerate for at least a few hours before serving.
Keeps fresh for weeks if stored in the refrigerator.
Notes
Ginger Cooking Time: 1 minute is for spicy ginger, the longer you cook it, the less spicy it gets. Don't cook longer than 5 minutes.
Eaten up all the ginger? Don't spill the vinegar mixture. Keep it in the fridge and use it next time to prepare a stir-fry sauce, mixed with soy sauce and a little brown sugar.