Homemade Tonic Syrup

zest from 3 limes
zest from 3 lemons
zest from one grapefruit
1 cup sugar
3 cups water
1 1/2 tablespoons cinchona bark
1 teaspoon gentian root
1 1/2 tablespoons citric acid
2/3 cup chopped lemongrass
1/8 teaspoon kosher salt
2-3 cups rich simple syrup or other sweetener equivalent.

Optional spices:
6 juniper berries
4 whole cardamom pods
6 whole allspice berries
2 teaspoons lavender
1 piece star anise
1/2 teaspoon quassia bark

For dry tonic, omit simple syrup and reduce citric acid to 1 teaspoon.

Stir zest and sugar together, let sit, covered, overnight. In a saucepan, bring all ingredients except the simple syrup to a boil and reduce heat immediately; simmer on low for a half-hour, then remove from heat, add to zest mixture and allow to cool fully. Chill overnight. Strain through a fine-mesh strainer. Return to the jar and refrigerate for a day or two, allowing sediment to accumulate on the bottom.

When the sediment layer seems stable, gently pour off the clearer liquid and discard sediment. Add simple syrup until it's as sweet as you like (2 to 3 cups.) Other liquid sweeteners such as agave or cane syrup or stevia can be substituted.

Funnel into a clean bottle, cap tightly, and refrigerate. Keeps in the fridge 2 to 3 weeks or freeze to store longer. Makes roughly 2 pints.  To use, mix 1 ounce of syrup with 5 to 6 ounces of club soda.  If making mixed drinks, the bubbles will last longer if you mix the syrup with alcohol first, then add the club soda last.