Remove 1/8 inch of the blossom end of each cucumber (this is the end opposite of the stem that attached the cucumber to the plant. This part contains enzymes that can make your pickles soft).
Cut cucumbers into even-sized slices (I use a mandolin) and pack into jars, with a layer or two of thinly sliced onions (and optional jalapeño slices). Fill to the top!
In a saucepan, combine the water, vinegars, sugar, salt, and pickling spice. Bring to just a boil, stirring occasionally to make sure all the salt and sugar dissolves. Pour the hot brine into the jar over the cucumbers.
Let them cool on the counter for an hour and then put a lid on and transfer to the fridge. Make sure that the liquid is completely covering the cucumbers. If after an hour its not, mix together 2 parts water to 1 part vinegar- just enough to top them off (no need to heat this).
Vinegar will make your metal jar lids and bands rust, which is why I like to use plastic screw covers for things like this.
It is best to wait a week before digging into your bread and butter refrigerator pickles, and they will last a couple months in the fridge.