Begin by making lemon-rosemary simple syrup. In a small saucepan, combine lemon juice, sugar, water, and 2 rosemary sprigs. Simmer gently over low heat, stirring occasionally, until sugar has completely dissolved.
Continue to simmer for another 2 minutes. set mixture aside to cool. Discard rosemary sprigs.
To serve, fill glasses with ice. add 2 parts lemon-rosemary syrup to one part seltzer + a touch of vodka, if desired.