Scrape the 2 teaspoons of zest from one of the lemons, then cut all of the lemons in half and juice them. Measure 1 cup, reserving the rest for another use.
Use a melon baller to remove any remaining pulp. Slightly trim the bottom of each lemon cup so it sits flat, then set them on a plate, cover, and place them in the refrigerator.
In a medium saucepan, combine the water and sugar over medium heat, stirring until the sugar has dissolved. Bring the water to a boil, then add the lemon juice and zest and boil for 2 minutes.
Transfer the liquid to a mixing bowl and allow it to cool to room temperature. Place the bowl in the freezer until the mixture begins to freeze, several hours or overnight. Use a fork to break up the ice, then stir it until fluffy.
Spoon 1/2 cup of the slush into each lemon half, then cover them and freeze until you're ready to serve. Garnish with a mint leaf and lemon zest before serving, if you like. Serves 8 to 10.