In a small saucepan over medium-high heat, bring 1/2 cup sugar and 1/2 cup fresh lemon juice to a boil. Add 2 Tbsp. roughly chopped rosemary leaves. Reduce heat to maintain a simmer and cook until slightly thickened, about 10 minutes. Add the zest of 1 lemon. Transfer syrup to a small metal bowl and let sit until cool, about 10 minutes. Strain (discard the zest and rosemary) and use warm, let cool to room temperature, or store, covered and chilled, up to six months.