Add the filtered water, thyme and lemon zest and lemon juice to a medium sized sauce pan, and bring the combination to a boil. Do not use a small sauce pan or you may have trouble with the process.
Once the water has come to a boil, cover the pan and remove from heat. Allow the pan to sit undisturbed for about 30 minutes so the thyme and lemon peel can fully steep.
Strain the thyme and lemon zest from the liquid using a fine mesh strainer.
Add the honey to 1 cup of the lemon-thyme herbal infusion in your medium-sized sauce pan and bring the mixture to a boil, then reduce the heat to medium. You’ll need to stir constantly until the temperature reaches 300 degrees F (hard-crack stage). It should take about 30 minutes to reach this point. You can test to see if it's done by putting a drop of the mixture into a cup or bowl of ice water. If it immediately hardens, it’s done. Living at a high altitude water boils at a different temperature, so this is how I tell when my DIY cough drop mixture is done.
Once it’s done, immediately remove the pan from heat and stir until the bubbles dissipate. Pour the mixture into your silicone molds. You'll want to do this ASAP so that the mixture doesn't harden in the pan.
Allow the cough drops to cool completely. This could take a few hours.
Once your cough drops are completely cooled, coat them in slippery elm powder to boost their soothing power and keep them from getting sticky in storage. I keep my DIY Cough Drops in the refrigerator to keep them nice and hard, and so they don’t stick together. You can wrap them in wax paper if you don’t want to refrigerate them.