1/2 cup freshly squeezed lime juice (from 4-5 standard-sized limes, more if using key limes)
1 cup sugar
3 branches fresh spearmint
1 2-liter bottle seltzer water, cold
A pinch of coarse salt
1. Find a pitcher that will hold 2 1/2 quarts of liquid. (If you don't have a large enough pitcher, you can scale the ingredients down accordingly.)
2. Make mint simple syrup: Put the sugar in a medium saucepan. Add mint and 1 cup cold water, stirring to combine. Heat the mixture over medium-low, stirring occasionally, until the sugar has melted, then turn the heat to low and continue to steep for 15 minutes. Remove from heat.
3. Remove and discard the mint and allow the sugar syrup to cool to room temperature. (If you're in a hurry, you can speed this process by cooling it in an ice water bath like this: Choose a medium bowl that will fit inside a larger bowl. Put ice water in the bottom of the larger bowl. Pour the syrup into the medium bowl and set the bowl inside the larger bowl so that the ice water comes partially up the sides. Adjust the amount of ice water to be sure no water will get into the sugar syrup. In about 10-15 minutes, the syrup should be cool enough to work with.)
4. Combine the lime juice and cooled mint syrup in the pitcher and pour in the seltzer along with a very small pinch of salt. Stir to combine.
5. When serving, add the ice to the glasses rather than the pitcher (ice will dissipate the bubbles faster, and can dilute the drink).
You can make the mint syrup ahead of time and store it covered in the refrigerator for several days. You can also freeze it for months.
To prep several pitchers ahead of time for a party, combine several batches of lime juice, mint syrup and a small pinch of salt in covered containers - each container holding enough for one pitcher. Then at party time, simply pour the contents of each container into a pitcher, fill with seltzer and you're ready to go.