Wash 1 lemon hot, rub dry and grate half of the peel. Halve the lemons and squeeze the juice. Pour the grated lemon with cane sugar and 75 ml water into a pot and stir over medium heat until the sugar is dissolved. Leave to cool in the refrigerator.
Mix coffee with tonic water, lemon syrup and 250 ml lemon juice and pour into glasses. If you like, you can freeze the remaining coffee in ice cube moulds and add it to the drink.