Bring about 4 cups of water and lemon zest in a pot over medium-high heat.
Lower temperature to a simmer, add 3 rosemary sprigs, and 10 Stevia packets. Allow to simmer for several minutes, stirring occasionally. This creates a rosemary simple syrup. Let cool.
Strain the liquid into the pitcher and add about 6 - 8 more cups of water and 5 more Stevia packets.
Chill in the refrigerator for several hours, or overnight. Cut extra rosemary sprigs into pieces that can be used a garnish.
Notes
Keep some extra sweetener on hand in case you decide to adjust the sweetness.