Combine 2 cups water and the cinchona bark in a small saucepan. Add the chopped lemongrass, citrus zests, bitter orange peel, coriander seeds, crushed cardamom pods, allspice berries and juniper berries (if using). Bring to a gentle simmer over medium-low heat, then reduce heat to low and maintain a bare simmer for 10–15 minutes, stirring occasionally. Do not boil aggressively (cinchona can foam and become bitter).

Steep & Strain

Remove the pan from heat and allow the mixture to steep, covered, for 30 minutes to extract aromatic and bitter compounds. After steeping, strain the liquid through a fine-mesh sieve lined with cheesecloth into a heatproof bowl to remove solids. Press gently on solids to extract as much liquid as possible, then discard solids (reserve if you wish to re-infuse lightly).

Add Citrus Juice

Stir in the juice of 1 lemon and 1/2 lime into the warm strained infusion.

Make Simple Syrup

In a separate small saucepan, combine 1 cup granulated sugar and 1 cup water for the syrup. Heat gently over medium heat, stirring until sugar fully dissolves and the mixture is clear — do not boil vigorously. Remove from heat as soon as dissolved.

Combine & Adjust

While the simple syrup is still warm, whisk it into the strained cinchona-citrus infusion until fully combined. Taste carefully (a small taste only — tonic syrup is concentrated). If the syrup is too bitter, add up to 2 tbs more sugar dissolved in a bit of warm water to balance. If too sweet, add a splash more lemon or lime juice. Allow the combined syrup to cool to room temperature.

Final Strain & Bottle

For a clearer tonic syrup, pass the cooled syrup through a fine mesh sieve lined with coffee filter or cheesecloth one more time into a clean bottle or jar. Refrigerate. The syrup will keep 2–3 weeks refrigerated.

Bottling & Serving

Mixing: For a single serving, fill a glass with ice. Pour 120 ml (about 4 fl oz) chilled club soda into the glass. Add 20–30 ml (about 1–1.5 tbsp) tonic syrup to start (ratio depends on desired bitterness). Stir gently and taste. Typical mixing ratios range from 1:5 to 1:3 (tonic syrup to soda) by volume; adjust to preference. In this recipe the suggested 120 ml syrup quantity refers to a batch amount — use small measured amounts when serving to avoid over-concentration.

Garnish & Serve: Garnish with a lemon or lime wheel. Serve immediately. Keep unused tonic syrup refrigerated and use within 2–3 weeks; dilute with chilled sparkling water when ready to drink.

Scaling: This concentrate yields roughly 300–350 ml of tonic syrup depending on evaporation and yields. Dilution guidance: start with 1 part tonic syrup to 4–5 parts sparkling water and adjust to taste.