Use a crinkle cutter or knife to cut cucumbers into ¼-inch slices
Slice onion into ¼-inch slices and place in a bowl with the cucumbers
Toss with kosher salt and set aside for 30 minutes
Meanwhile, place white vinegar in a bowl and set aside
In a saucepan, combine apple cider vinegar, brown sugar, coriander seeds, garlic cloves, mustard seeds, and optional red pepper flakes
Heat over medium, stirring until the sugar is dissolved
Remove from heat and pour mixture into the white vinegar
After 30 minutes, rinse salted cucumbers and onions, then place in a clean bowl
Pour the vinegar mixture over the cucumbers and onions, stirring to coat evenly
Transfer to jars or an airtight container and refrigerate for at least 24 hours before serving