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
bottom of 5 pint jars. Pack the cucumbers lengthwise in the jars.
Combine the mustard seed, vinegar, sugar, and salt, bring to a boil
for 1 minute. Fill the jars to overflowing with the boiling--hot
syrup and seal.