Wash the cucumbers, quarter them lengthwise, and soak them in ice-water for 2 hours. Peel and slice the onions and pack them in the bottoms of 5 pint jars. Pack the cucumbers lengthwise in the jars. Combine the mustard seed, vinegar, sugar and salt, bring to a boil and boil for 1 minute. Fill the jars to overflowing with the boiling-hot syrup and seal. Makes 5 pints.