F&W's Grace Parisi makes this soda with iSi's new Twist 'n Sparkle soda maker, which carbonates beverages in less than 30 seconds. $50; williams-sonoma.com. More Nonalcoholic Drinks
In a medium saucepan, combine the sliced ginger with the smashed lemongrass and 4 cups of the water and bring to a boil over high heat. Stir in the sugar. Cover and let the ginger-lemongrass syrup steep off the heat until cool, about 2 hours.
Strain the syrup through a fine-mesh sieve. Discard the ginger and lemongrass. Stir in the lemon juice and the remaining 2 cups of water. Refrigerate until chilled.
Make the soda according to the manufacturers' directions. Serve in tall glasses over ice, garnished with lemon wheels and lemongrass stalks.
The recipe can be prepared through step 2; refrigerate up to 5 days.