Place the lemon zest on a cutting board and mound 1 teaspoon of the sugar on top. Cut the zest and sugar together with a knife, alternately chopping and rubbing the mixture with the flat side of the knife. Gradually the mixture will transform into a smooth paste.
In a 2-quart (1.9L) or larger saucepan, combine the zest paste, remaining sugar, lemon juice, butter, and agar agar. Bring to a boil over medium heat, whisking continuously and taking care to get into the edges of the pan bottom with your whisk. After 4 to 5 minutes, reduce the heat to medium-low and simmer for 2 to 3 minutes, until you are sure the agar agar is dissolved. Pour the mixture into a heatproof bowl, whisk in the sweetened condensed milk, and let cool at room temperature for 10 to 15 minutes. Cover and refrigerate for at least 2 hours, but preferably overnight. It will keep in the refrigerator for up to 1 week.
Genius Tip: Agar agar is a handy thickening agent derived from algae, and a vegan alternative to gelatin. Different brands can vary in their thickening power, but if your curd comes out too thick, simply rewarm it and whisk in a little water.