Add the 20g of dried hibiscus flowers and 500ml of water to a medium saucepan and set it over medium heat, which takes about 5 minutes. Watch closely, as the water turns a deep red within the first minute from the hibiscus anthocyanin pigments, and the moment it reaches a gentle simmer — small bubbles at the edge, no rolling boil — remove the saucepan from the heat. Let the flowers steep off the heat for 10–15 minutes: at 10 minutes the concentrate is brighter and more delicate, and at 15 minutes it is deeper in colour and more assertively tart. That just-to-simmer, off-the-heat steep is what keeps the volatile floral aromatics in the liquid rather than driving them off into a cooked, jammy flatness.

While the concentrate is still hot from steeping, stir in the 60g of honey until it has dissolved completely and no streaks remain at the bottom of the pan. The residual heat is enough, so the saucepan does not go back on the burner. Leave the sweetened concentrate to cool for 5–8 minutes, until it reaches roughly 50–60°C and feels warm to the touch but no longer hot. That warm stir-in is what gives even sweetness throughout, because honey added to cold liquid sinks and dissolves in pockets, and it avoids the extra heating that would dull the hibiscus.

Once the concentrate has cooled to roughly 50–60°C, add the zest of ½ lime and let it infuse for 5–10 minutes, stirring once or twice, until the liquid smells clearly of lime above the hibiscus. At this temperature the zest’s aromatic oils release into the liquid but do not evaporate on contact. That is what gives the concentrate a fragrant citrus thread that is separate from the sharp acidity of the juice, since the zest provides the fragrance and the juice provides the brightness.

Leave the concentrate to cool fully to room temperature, which takes a further 10–20 minutes, until the side of the saucepan feels neutral to the touch. Only then stir in the 60ml of fresh lime juice and taste, looking for a vivid, tart, floral concentrate with the honey rounding the edge and the lime sitting clearly on top. If it still feels warm, wait a few minutes more rather than adding the juice early. That room-temperature addition is what preserves the juice’s clean acidity, because warm liquid dulls fresh lime within the first minute of contact.

Strain the concentrate through a fine-mesh sieve into a clean jug, pressing gently on the flowers and zest to extract as much liquid as possible, and discard the solids. You should have roughly 500ml of concentrate, of which the four glasses use about 320–340ml. Cover the jug and refrigerate for at least 30 minutes, until thoroughly cold. That thorough chill is what stops the concentrate from melting the ice and flattening the club soda on contact.

Fill four large wine glasses generously with ice cubes, then divide the chilled concentrate evenly among them, about 80–85ml per glass, and stir each briefly with a long spoon. Top each glass with about 125ml of chilled club soda, pouring gently down the inner side of the glass, and stir once or twice. Press a thin lime slice against the inside of each glass so it shows through the ruby liquid, and float a dried hibiscus flower on the ice if using. Keep the leftover concentrate, roughly 150–200ml, refrigerated. Serve immediately. That gentle pour, with the club soda added only at the glass, is what keeps the carbonation at full intensity.