Wash vegetables; cut cucumbers in 1/2-inch slices without paring. Peel onions and cut in 1/4-inch slices.

Place layers in a bowl with salt; let stand 3 hours. Drain and rinse well with cold water.

Mix water, sugar, mustard seed, and celery seed in a large kettle. Boil until sugar is dissolved.

Add cucumber and onion and boil 5 minutes.

Pour into hot, sterile jars and seal immediately.