Put the lemon juice, sugar and half the mint leaves in a cocktail shaker. Bash with a cocktail muddler or the end of a rolling pin until the sugar has dissolved.

Pour the rum and limoncello into the shaker and add a handful of ice. Cover with the lid and shake until the outside of the shaker feels cold.

Fill two tall glasses with ice, then strain in the cocktail and top up with the soda water. Garnish with the remaining mint leaves to serve.