Place the cucumbers and onions in a large bowl and cover with the salt water. Cover the bowl with foil or plastic wrap and refrigerate overnight.
The next day, drain the cucumber-onion mix in a colander and RINSE WELL with cold water for approximately 5 minutes, turning the mixture with your hands as you rinse.
After 5 minutes, taste a cucumber. If it’s very salty, but not inedible, it’s ready.
Pour the cucumber-onion mix onto several folded paper towels and pat as dry as possible—but don’t squeeze! Place the mixture in a clean bowl and add the rest of the ingredients (add the jalapeño slowly and taste for heat). Best made at least a few hours ahead of eating.