Combine cucumbers and onion; sprinkle with salt, let stand 1 hour.
Combine sugar, vinegar, garlic (if using); pour over cucumber mixture.
Refrigerate 1 day.
Drain off most of brine.
Place in containers, leaving 1" head space; freeze.
NOTE: can be stored in fridge for 2 weeks or frozen. To serve, thaw unopened in refrigerator for 6 hours.