First cut the limes in half and squeeze them properly. Pour the juice through a sieve into a saucepan and mix with the sugar and elderflower syrup.
Heat the liquid slowly over medium heat and stir constantly until the sugar has dissolved. Add the mint and slowly bring the syrup to the boil, stirring constantly. The longer you let it simmer, the more intense you will taste the mint afterwards. Take the saucepan off the stove and remove the mint leaves.
Pour the liquid into a container and let it cool down, then store it in the refrigerator.
For a delicious lemonade, add a little syrup with ice-cold sparkling water. Especially delicious on hot summer days.