Slice and soak the cucumbers and onions for 1 hour in 1 cup salt and enough cold water to cover.
Make brine: Combine vinegar, water, sugar, garlic, peppercorns, allspice and mustard seeds. Bring to a boil and cook for 5 minutes. Drain the cucumbers and onions but don't rinse. Pour the hot brine over the cukes and onions and allow to sit in a nonreactive container until cool, stirring occasionally.
Meanwhile, wash and sterilize 4 1-quart jars and lids to be used in the canning by placing them briefly in a pot of hot water. When the pickled mixture has cooled, fill the jars to the top with the cucumbers and onions only, not the brine. Once you have filled all the jars, divide the brine between the jars, filling them to the top. With a hot clean towel, wipe the edges of the jars, secure the lids and screw on the rings. You will now need to process them for 8-10 minutes in a simmering water bath. It is important for the jars to be on a rack so they do not touch each other or the bottom of the pot. The jars must be completely covered by the water therefore use a huge pot. (Contact your local Ball Jar distributor for information on equipment: racks, tongs, even pots). Allow the water in processing bath to come to a boil, turn it down to a simmer and carefully place the jars on a rack inside of the pot. Simmer for 8-10 minutes. Carefully remove from the water, allow to cool, don't let the jars touch. When they are cool, they should have a tight seal, no sound should be heard when you push on the lid. Store for months on your shelf.