Add the sliced peaches to a medium saucepan — skins on intentionally. The skin of a ripe peach contains concentrated aromatic compounds and pectin alongside a deeper, more complex fruit character than the flesh alone; including the skins in the cooking liquid produces a syrup that is more intensely peach-forward with a naturally deeper colour than peeled-peach versions. Using a fork or potato masher, lightly mash the peach slices in the pan — not to a smooth purée but simply breaking each slice into rough pieces that will release their juice and interior flesh more readily during the simmer. The goal is maximum juice extraction rather than a smooth base. Add the 60g of light brown sugar and 240ml of water. Place over medium-low heat. Bring to a gentle simmer — not a rolling boil, which would cloud the syrup and produce a harsher extraction. Cook for 8–10 minutes, stirring occasionally, until the peach pieces have broken down significantly into the surrounding liquid, the sugar has fully dissolved, and the liquid is visibly golden-amber and fragrant. The brown sugar's molasses content contributes a subtle caramel warmth that white sugar would not — amplifying rather than simply sweetening the peach's own warm, slightly honeyed flavour.

Remove the saucepan from the heat. Add the 2 rosemary sprigs and the lemon zest from ¼ lemon immediately. Allow to steep off heat for 8–10 minutes. The off-heat steeping is the specific technique decision that makes the rosemary character in this syrup herbal and aromatic rather than piney and medicinal. Rosemary's dominant aromatic compounds — camphor, pinene, and camphene — are highly volatile. At gentle steeping temperature off heat they release gracefully into the warm liquid as clean herbal aromatics; at extended simmering temperature they over-extract and develop the sharp, resinous character that makes rosemary taste like something from a medicine cabinet rather than a herb garden. 8–10 minutes at off-heat steeping temperature extracts the aromatic oils at their most pleasant intensity. The lemon zest's volatile aromatic limonene infuses simultaneously, providing a bright citrus thread through the finished syrup that is fragrant rather than sharp.

Strain the syrup through a fine-mesh sieve set over a clean heatproof jug or bowl. Press gently on the peach solids and rosemary sprigs with the back of a spoon to extract as much syrup as possible before discarding the solids. The finished syrup should be clear to slightly cloudy, deeply golden, and specifically fragrant. Transfer to the refrigerator and chill until completely cold — a minimum of 30 minutes. A warm syrup added to iced club soda dilutes the ice and flattens the carbonation on contact. The fully chilled syrup added to cold club soda over ice preserves the carbonation and maintains the correct temperature balance of the finished drink.

Fill four tall glasses generously with ice cubes. Divide the chilled peach rosemary syrup evenly — approximately 45–50ml per glass. Swirl each glass briefly to chill the syrup fully and lightly coat the ice. Top each glass with approximately 125ml of chilled club soda, pouring gently down the side of the glass to preserve the carbonation rather than pouring directly into the centre which releases bubbles rapidly. Stir once or twice gently. Garnish each glass with a fresh peach wedge rested on the rim and a small rosemary sprig inserted alongside. Serve immediately while the carbonation is at its highest.