In a large pan, combine the onions, mustard seed, turmeric and celery seed.
In a saucepan, combine the sugar, vinegar and salt. Bring to a boil. Pour the boiling mixture into the pan with the onion mixture.
Add the cucumber slices. Pack into sterile jars. Refrigerate for 2 weeks before eating.
These pickles take two weeks in the refrigerator, but are well worth the wait.