2 cups water
1 cup granulated sugar
Zest from 2 limes, divided
1/2 cup fresh lime juice (about 3 to 4 medium limes)
1 tablespoon fish sauce
In a small saucepan, combine the water, sugar, and half of the zest. Cook over medium heat, stirring occasionally, until the sugar is dissolved. Remove the pan from the heat and cool completely. Stir the sugar water, lime juice, remaining zest, and fish sauce together, and then pour into an 8x8-inch baking pan (or similar-sized dish). Place in the freezer.
Scrape the mixture with a fork to break it up into smaller crystals. Return to the freezer, and repeat this process every hour or so, until you have a frozen but grainy mixture, about 3 hours total. If you forget to stir the mixture and it freezes solid, break it into large chunks and pulse in the food processor to bring it back to a granular, flaky texture.
To serve, spoon the granita into chilled glasses.