Using a crinkle cutter, slice the cucumbers into ¼-inch rounds.
Cut the onion into ¼-inch slices and combine with the cucumber slices in a large bowl.
Toss everything with the kosher salt and let sit for 30 minutes.
While the cucumbers rest, pour the white vinegar into a bowl and set aside.
In a saucepan, combine the apple cider vinegar, brown sugar, coriander seeds, garlic cloves, mustard seeds, and red pepper flakes (if using).
Heat over medium heat, stirring occasionally, until the brown sugar completely dissolves.
Remove from heat and pour the mixture into the bowl with the white vinegar.
After the cucumbers and onions have sat for 30 minutes, rinse them thoroughly and transfer to a clean bowl.
Pour the vinegar mixture over the vegetables, ensuring everything is fully submerged.
Cover and refrigerate for 24 hours before serving.
Notes
Crinkle cutter creates classic pickle look but regular knife works fine. Make sure cucumbers are completely covered by brine. Flavor improves after 24-48 hours.