1 1/2 cups cider vinegar
1/2 cup brown sugar
1 tablespoon crushed red pepper flakes
1 tablespoon celery seeds
1 tablespoon coriander seeds
1 tablespoon mustard seeds
1 tablespoon minced fresh dill
One 12-ounce cucumber, sliced into thin discs
1 jalapeño, seeded and sliced thinly, optional
1/2 small onion, thinly sliced
Combine the vinegar, sugar, crushed red pepper flakes, seeds and dill with 1/2 cup water in a small saucepan. Bring to a boil, then remove from the heat.
Toss together the cucumber, jalapeño if using and onion, then transfer to a wide-mouth quart size Mason jar. Carefully pour over the warm vinegar mixture.
Cover and refrigerate for at least 12 hours or up to 7 days.