Place the vinegar, water and salt into a medium sauce pan. Cook over medium-low heat until the salt and sugar has dissolved. Stir occasionally.
Place the mustard seed, red pepper flakes, onions, garlic cloves and cucumbers into a large canning jar or two canning jars. Pour the vinegar mixture over the cucumbers. Bring the pickle mixture to room temperature.
Cover with the lid and store in the fridge up to two months.
Notes
This recipe can be easily doubled and tripled. If you prefer the pickles to be less spicy, reduce the amount of red pepper flakes.