1. Combine ingredients in a medium saucepan and bring to a boil over high heat.
  2. Once mixture starts to boil, reduce heat to low, cover and simmer for 20 minutes.
  3. Remove from heat and strain out solids using a strainer or chinois.
  4. You’ll need to fine-strain the mixture, as it still contains quite a bit of the cinchona bark.
  5. You can use a coffee filter and wait for an hour or more, or do as I do and run the whole mixture through a French coffee press.
  6. Once you’re satisfied with the clarity of your mix, heat it back up on the stovetop or microwave, and then add ¾ cup of agave syrup to each cup of your hot mix.
  7. Stir until combined, and store in the attractive bottle of your choice.
  8. You now have a syrup that you can carbonate with seltzer water; I use my iSi soda siphon for some nicely-textured bubbles.
  9. To assemble a gin and tonic, use ¾ ounce of syrup, 1½ ounces of gin and 2 ounces of soda water over ice.