Halve the oranges and limes and use a juicer to extract the juice. Wash the elderflower and drain on kitchen paper.
Measure out 500 ml of juice. If it is not enough, top up with water.
Put the juice with the elderflower in a saucepan and cook for about 10 minutes. Allow to cool slightly. Then strain the juice through a sieve with cheesecloth into a second saucepan. Wring it out well. Mix this well with 500 g sugar and simmer on medium heat for about 6 to 8 minutes.
Rinse a clean bottle with hot water and pour the syrup through a funnel.
The syrup can be kept in the refrigerator for about 1 week. This recipe makes about 1 liter.
If you want to do more, you can keep the syrup in the canning machine at 80 degrees for about 30 minutes after filling. Then it should hold up well for about 6 months.