Larousse. They make it with vodka, but as you know, I'm a gin martini person.
Peel & roughly chop half a cucumber, you are going to muddle this with some sugar in the bottom of your mixing glass to get as much liquid out of the cucumber as possible. Having successfully muddled add a dash of dry vermouth (to counter the sugar), a very generous handful of ice and as much gin as you need(*) stir down for a good 30-40 seconds then double strain to serve.
Drink, very very carefully. On a hot day the refreshing nature of ice-cold cucumber will dominate pretty much everything, don't be fooled, there is a good dose of hard liquor in there which ever way you choose to make this one.
(*) Taking into account the number of them you are making (half a cucumber will do upto 4) and how strong/dry you want the final drinks to be.