Wash cucumbers well. Slice, then discard any slices with blemishes.
Mix cucumbers, onions, and salt; let stand 2 hours. Drain and rinse off excess salt.
Combine vinegar, boiling water, mustard seed, celery seed, turmeric and sugar. Bring to a boil, add cucumbers. Cook 5 minutes.
Pack into hot sterilized jars leaving 1/4-inch headspace. Wipe rim of jar with a damp paper towel. Adjust 2 piece caps. Process in a boiling water bath canner for 15 minutes.
*Test cucumbers for hollowness: if a cucumber floats in a bucket of water, it's a "hollow" cucumber and is good for relish, not pickling.
**To avoid a cloudy brine or darkened pickles, do not use common table salt (use only canning salt).