2 ½ cups apple cider vinegar
2 ½ cups sugar
2 ½ tablespoon mustard seeds
2 ½ tablespoon Kosher Salt
1 medium onion, sliced into half-moons
2 ½ pounds cucumbers, sliced
In a large non-reactive saucepan, bring the cider to a simmer and add the sugar, mustard seed, Kosher salt. Simmer for about 3 minutes.
Divide the onions and cucumbers among several 1-quart canning jars and cover them with the cooking liquid. If there is not enough liquid, add a little white wine vinegar to cover. Allow the jars to cool, cover with lids, and refrigerator.
Refrigerate for 2 weeks, or to taste.