Rinse cucumbers under cold running water and drain. Remove the ends of each cucumber, and cut into ¼ inch slices. Cut onion into thin slices. Place cucumbers and onions in a large bowl and mix.
Sprinkle canning salt over the mixture and cover with a clean tea towel. Scatter ice over the tea towel, and let stand for 2½ hours.
Pre-heat jars
30 minutes before the cucumbers are ready, place jars on the rack of your water bath canner. Fill the canner with water, covering the jars by 2 inches. Bring water to 180℉, and simmer jars for at least 10 minutes. Leave jars simmering until ready to fill.
Fill the jars
After 2½ hours, rinse cucumbers and onions under cold running water to remove salt, and allow to drain.
Meanwhile, combine the white vinegar, apple cider vinegar, and sugar in a stockpot. Bring mixture to a boil, stirring until the sugar dissolves.
While the pickling liquid comes to a boil, remove the jars from the canner and place them on your tea towel.
Working quickly, add 1 tsp mustard seed, ¼ tsp celery seed, ¼ tsp allspice berries, and ⅛ tsp turmeric powder to each hot jar. Then, pack the cucumbers and onions into the jars, leaving ½ inch headspace.
Pour or ladle the hot pickling liquid over the packed jar. Leave ½ inch headspace. Wipe the jar rims with a clean, damp cloth and center clean, new lids on the jars. Then, screw on the bands to fingertip-tight.
Process the jars
Place the jars in the simmering water bath canner one at a time using the jar lifter. Water must cover the jars by 1 inch. Cover the canner and adjust the heat to medium-high to bring the water back up to a rolling boil. Allow the jars to process in the boiling water for 10 minutes, adding additional time for your altitude (see table above).
After 10 minutes, turn off the heat, remove the cover, and let the boil stop.
Carefully remove the jars from the canner and place on a towel. Let them fully cool undisturbed for 12 hours.
Testing the seals
After 12 hours, test the seals by pressing the center of each lid to see if it is flat. Double-check your seals by removing the bands from the jars and holding each jar up by the edge of the lid for 5 seconds. If the seal does not hold, store that jar in the refrigerator and eat the pickles within 4 weeks.
Wipe your jars with a clean, damp cloth to remove any sticky residue from the canning process. Label each jar with the date and store them in a cool, dry, dark place for up to one year.
Notes
Your pickles should reach their full flavor within 4 weeks, but can be kept for up to one year. When you do decide to open a jar of these sweet bread and butter pickles, keep the jar in the fridge and consume the pickles within 6-8 weeks.