Gently stir salt into thinly sliced cucumbers. Cover with ice cubes; let stand 2 or 3 hours or until cucumbers are crisp and cold. Add more ice if it melts. Drain; add onions.
Combine remaining ingredients; bring quickly to a boil and boil 10 minutes.
Add cucumber and onion slices and bring to boiling point. Pack at once in hot jars. Adjust lids. Proces