Combine sliced cucumbers, onions, and salt in a large bowl.
Cover with ice water and let stand for 2 hours.
Drain well and rinse lightly.
In a large saucepan, combine both vinegars, sugar, mustard seeds, celery seeds, turmeric, and peppercorns.
Bring the brine to a gentle boil while stirring until the sugar dissolves.
Add the cucumber and onion mixture to the hot brine.
Simmer for 5 minutes until the cucumbers are slightly softened but still crisp.
Transfer the pickles and brine into clean jars.
Cool completely before sealing.
Refrigerate for at least 24 hours before serving for the best flavor.