Add the granulated sugar and whisk again.

Heat the mixture over medium heat until it comes to a simmer, stirring occasionally.

Reduce the heat to low and continue to cook for 2-3 minutes until the agar agar and sugar are completely dissolved.

Remove the pan from the heat and stir in the lemon juice and lemon zest.

Pour the mixture into the prepared baking dish and let it cool to room temperature.

Once the mixture has cooled and set, cut it into cubes and serve.