Scrub cucumbers and thinly slice without peeling. Alternate cucumbers and onion slices in a bowl. Sprinkle a little of the salt between each layer. Leave 30 minutes.
Meanwhile, mix remaining ingredients and bring to a boil. Simmer 5 minutes.
Rinse cucumber and onion slices under cold water to remove salt and drain. Add cucumbers to boiling syrup and boil for 1-2 minutes.