Slice the citrus into 1cm slices and arrange on 2-3 roasting trays lined with baking paper. Scatter over the cherries and then the herbs and spices.
Roast at 220°C (200°C fan-forced) for 1-1.5 hours, swapping trays half way, until the citrus becomes blackened at the edges.Bring the sugar and water to a simmer, stirring until the sugar is completely dissolved, then allow to cool to room temperature.
Transfer the contents of the roasting tray to a large preserving jar and pour over the sugar syrup. Allow to steep at least overnight but preferably for a week or two, then strain the syrup into bottles.
Serve over ice, diluted with soda water.
