Add the lemon juice to a pitcher with the lime juice and zest and sugar. Stir well until the sugar is dissolved.
Top up with plenty of ice and about a litre of tap water or sparkling water. Mix well and add more sugar, to taste.
Add the mint sprigs, if using, to the pitcher and stir well.
For an even lemonier lemonade, peel the zest from the lemons in strips with a sharp knife and add this to the pitcher.