Add the granulated sugar, lemon zest, and lemon juice to a medium-sized saucepan. Heat over medium heat, whisking continually, or until the sugar has dissolved but isn’t simmering yet, or has reached around 140-150 degrees Fahrenheit.
Increase the heat to medium. Cook for 2-5 more minutes, or until the mixture thickens, gets slightly foamy, and reaches 150 degrees Fahrenheit. Remove the pan from the heat.