Start by washing your cucumbers well and slicing off both ends before cutting them into even ¼-inch rounds. Slice your onions just as thin so they cook and pickle at the same rate as the cucumbers. Keeping the slices consistent in size really does make a difference in the final texture.
Place the sliced cucumbers and onions in a large bowl and sprinkle the salt evenly over the top. Toss everything together so the salt coats every piece, then cover the bowl and let it sit in the refrigerator for a few hours. This step draws out excess moisture, which is the secret to a crisp, not soggy, pickle.
Once the vegetables have chilled, pour them into a colander and rinse under cold water to remove the extra salt. Give them a good shake to drain off as much water as possible. Skipping this step can leave your pickles tasting overly salty, so don't rush through it.
In a saucepan, combine the vinegars, sugar, and all of the spices over medium heat. Stir occasionally and let the mixture come to a gentle simmer until the sugar fully dissolves. The kitchen will smell incredible at this point, so take a moment to enjoy it.
Pack the drained cucumbers and onions tightly into your jars, leaving a little room at the top. Ladle the hot brine over the vegetables, making sure everything is fully submerged. Leave about a half-inch of space between the brine and the rim of the jar.
Let the jars cool down to room temperature on the counter before sealing them with their lids. Once cooled, place them in the refrigerator and let them sit undisturbed overnight. This resting time is what allows the flavors to really soak into the cucumbers.