Wash cucumbers and onions. Slice cucumbers thinly and slice onions thinly. Combine sliced cucumbers and onions in a large bowl. Sprinkle with salt and toss to coat. Let stand for 2-3 hours at room temperature, stirring occasionally. Drain thoroughly and rinse well.
In a large pot, combine vinegar, sugar, celery seed, mustard seed, and turmeric. Bring to a boil over medium-high heat, stirring until sugar is dissolved. Reduce heat and simmer for 5 minutes.
Add the drained cucumbers and onions to the pot with the brine. Return to a boil, then reduce heat and simmer for 10 minutes, stirring occasionally.
Carefully ladle the hot pickles and brine into clean jars, leaving about 0.5 inch headspace. Seal the jars tightly.
Allow pickles to cool to room temperature, then refrigerate for at least 24 hours before serving. Pickles will keep in the refrigerator for up to a month.
Notes
For best results, use fresh, firm cucumbers. You can adjust the sugar and vinegar ratio to your preference.