Mix cucumber slices and salt together in a bowl; let sit for 1 1/2 hours.

Pour cucumbers into a sieve set in the sink. Thoroughly rinse cucumbers under cold running water and drain well.

Combine white sugar, white wine vinegar, apple cider vinegar, brown sugar, mustard seeds, celery seeds, and turmeric in a saucepan over medium heat. Bring to a boil, then cook and stir until sugar dissolves, about 3 minutes.

Place drained cucumbers into a heat-proof bowl. Add onion, then pour hot vinegar mixture over top. Let stand at room temperature for 1 hour.

Transfer cucumbers and liquid to a container with a tight-fitting lid. Cover and refrigerate for 24 hours before eating.