Slice the cucumbers and onion into thin slices In a large bowl, toss the cucumber and onion slices with the salt. Cover and chill in the refrigerator for 1 hour.
Transfer the cucumbers and onions to a colander and rinse off the salt. Drain excess water from the bowl and return the cucumber and onions to the bowl.
In a small saucepan, combine the remaining ingredients and heat over medium-high heat just until the sugar has dissolved stirring continuously.
Pour the vinegar mixture over the cucumbers and onions and allow to cool to room temperature. Once cooled, chill in the refrigerator for at least 24 hours.
Use them within two months while keeping them stored in the fridge.