Add the sugar-free lemon gelatin mix to a medium-sized bowl. Pour in 1/2 cup of boiling water and whisk until the gelatin dissolves.
Whisk in 1/2 cup of cold water. Let the mixture rest for 15 minutes to come to room temperature.
Add 1 cup of fat-free Greek yogurt to the lemon gelatin. Whisk together until there are no yogurt clumps.
Next, whisk in 1 teaspoon of lemon zest. Pour the mixture into two small bowls and refrigerate until set, approximately 2 hours.