Carefully rinse the cucumbers under running water, scrubbing to remove any dirt. Trim off 1/8 inch from both ends, then slice the cucumbers into 1/4-inch thick slices. Place these slices into a large bowl.

Add the thinly sliced onions and all the pickling salt to the cucumbers. Stir well to evenly distribute the salt. Cover the bowl with a clean, thin tea towel, then top with a couple of inches of ice. Refrigerate for 4 hours to draw out excess moisture and enhance crunch. After chilling, discard the ice and thoroughly rinse the cucumber and onion slices to remove excess salt. Drain well and rinse again before draining thoroughly.

If planning to store pickles outside the refrigerator, prepare your jars by placing them on a metal rack in a large 16-quart canning pot filled with warm water—jars should be submerged at least 1 inch. Bring water to a boil, then reduce heat to keep jars hot and ready. This step is essential for safe canning. If storing refrigerated only, this step can be skipped. Clean lids separately in hot, soapy water.

In a large 4 to 6-quart pot, combine the white distilled vinegar, apple cider vinegar, sugar, and all pickling spices except salt. Bring the mixture to a boil, stirring until the sugar dissolves. Add the rinsed cucumber and onion slices to this hot syrup and bring to a boil again.

Using a slotted spoon, pack the hot cucumber and onion slices into the hot jars, leaving 1 inch of headspace at the top. Pour the hot pickling syrup over the packed vegetables, filling to 1/2 inch from the rim. Wipe the jar rims clean with a paper towel, place dry clean lids on top, and secure with metal screw bands. Repeat for all jars.

If canning for shelf storage, lower the filled jars back into the canning pot with water covering the jars by at least 1 inch. Bring to a rolling boil and boil hard for 15 minutes to properly seal and preserve. Remove jars with tongs or jar lifters and allow to cool completely.

Allow jars to cool to room temperature; lids should make a popping sound as they seal. Unsealed jars must be stored in the refrigerator and consumed within 3 months. Properly sealed jars can be stored in a cool, dark place for up to 1 year. If you skipped the water bath, refrigerate the pickles and use within 3 months.