Zest the lemons and limes using a microplane or zester
Blanch the citrus in boiling water for 1 minute, then save the leftover liquid
Cut the lemons and limes in half and juice them
Add the sugar and lemon zest to the lemon water
Bring the mixture to a simmer
Add the lemon juice and bring to a simmer again
Strain the mixture to finish the cordial
Store the cordial in the fridge for up to 1 month
Mix the cordial with sparkling water to make a summer drink
