Wash the cucumber and cut into small cubes.
Peel the shallots and cut into thin slices together with the chili.
Put the cucumber, shallots and chilli in a large enough bowl and mix.
Heat vinegar with sugar and salt, stirring constantly until sugar is completely dissolved.
When the mixture comes to a boil, remove from heat and let cool.
Then pour over the cucumber and garnish the lettuce with the cilantro.