Originally, a recipe from Tyler Florence, but I found it here: http://mytastytreasures.blogspot.com/
ICY LEMON-GINGER COCKTAILS
4 teaspoons grated fresh ginger
2 cups water
1 1/2 cups sugar
finely grated zest of one lemon
1 cup fresh lemon juice
8 cups crushed ice
2 cups vodka
In a small saucepan, combine the ginger with the water and sugar and bring to a simmer over moderate heat. Simmer for 5 minutes, then let the ginger syrup cool slightly. Transfer to a blender and puree. Strain the syrup into a shallow glass dish (about 9 x 9 square). Stir in the lemon zest and juice. Cover and freeze the lemon-ginger syrup until firm, about 4 hours.
Let the frozen lemon-ginger mixture stand at room temperature for 5 minutes. Chop the mixture into large pieces. Put half the pieces in a blender. Add 4 cups crushed ice and 1 cup vodka. Blend until slushy. Serve. Repeat with remaining mixture.
Yield: Makes about 6 drinks if you use tall glasses.
Note: The lemon-ginger mixture can be frozen for up to 1 week.