In a non-aluminium saucepan, bring the water to a boil. Pour over the rosemary and let steep for 1 hour. Strain.
Remove the zest from 5 of the oranges and set aside, then squeeze the juice into a large bowl. Set a rack over the bowl and arrange the zests on the rack. Combine the rosemary infusion and the sugar mixture in a saucepan and bring to a boil over high heat, boiling until you have a heavy syrup about 20 minutes. Pour the boiling syrup over the zests and let it drain into the juice. Return the juice mixture to the saucepan. Bring quickly to a boil, remove from the heat, and return to the bowl to cool. Skim, then bottle and store in the refrigerator.
Use this syrup with fruits, in a marinade for chicken or pork, or as the basis of a cooling drink. Just add water or sparkling water to dilute.
MAKES: 9 CUPS