Carefully rinse the cucumbers, taking care to clean the skins well. Slice off the very ends of each cucumber and discard. Slice the cucumbers in 1/4-inch thick slices, place in a large bowl or roasting pan. Add the sliced onions and kosher salt. Stir in so that the salt is well distributed among the cucumber slices. Cover with a clean tea towel (not terry cloth) and then cover with a couple of inches of ice. Put in the refrigerator and let chill for 4 hours. Discard ice. Rinse the cucumber and onion slices thoroughly, drain. Rinse and drain again.
If you are planning to can the pickles, follow the below instructions. If you are planning to just store the pickles in the refrigerator, you can skip this step. Wash the canning jars and lids with warm soapy water. Then fill each of the jars with boiling water right to the very top of the jar. I usually put the jars on a wooden cutting board before I add the boiling water. Leave the boiling water in the jars until you are ready to start filling with the cucumber mixture. Sterilize the lids by putting them in a pot of water and boiling for a few minutes. Leave the lids in the boiling water until you are ready to use them. Also at this time, fill a canning pot with water and bring to a boil. Once this boils, leave on low heat until you need it later in step 4.
In a 6 quart pot, add the vinegars, sugar, and all of the spices. Bring this mixture to boil. Once the sugar has dissolved, add the sliced cucumbers and onions. Bring to a boil again. As soon as the sugar vinegar solution begins boiling again, use a slotted spoon to start packing the prepared jars (if canning) with the cucumbers. Dump the boiling water out of the canning jar (one at a time) and pack the jar to an inch from the rim with the vegetables. Then pour hot vinegar sugar syrup over the vegetables to a half inch from the rim. Wipe the rim clean with a paper towel. Take a sterilized lid from the pot of boiling water and place on the jar. Secure with a metal screw band. If you are not canning, you can cool the boiling pickle mixture slightly and then put in any container you desire and store in the refrigerator.
If you are canning, continue on with this step. process the filled jars in a hot water bath for 10 minutes. Put the filled jars into the canning pot of boiling water (filled in step 2). The water level needs to be at least one inch above the top of the cans. Bring to a boil and let boil hard for 10 minutes. Remove jars from pot. Let cool down to room temperature. Jars should make a popping sound as their lids seal. If a lid doesn't properly seal, do not store the jar outside of the refrigerator.