Boil one quart of water and the rock sugar to make a syrup.
Rinse the lemongrass, peel off the skin, mash it with a knife or a pestle, and finely cut it into pieces.
Add the powdered ginger, smashed lemongrass, and salt into the boiling water and keep it going for five more minutes.
Turn off the heat and let it steep for 30 minutes.
Strain the liquid and let it cool.
Squeeze the lime juice into the mixture and stir.
Serve it with ice, garnish with a lemon slice and mint leaves, and enjoy.