To a cocktail shaker, add lime juice, tequila, cucumber and sweetener (if using).
Muddle everything together completely until cucumber juices have released. Really mash everything up completely!
Add jalapeño slices to the shaker along with a handful of ice, and shake to incorporate. The longer you shake the spicier the result.
Pour and strain into a glass with ice, and top with sparkling water.
Garnish with additional cucumber and jalapeño slices if desired. Enjoy!