Using a sharp knife or mandoline, thinnly slice cucumbers and onion ¼" thick, or thinner as desired.
Place the onion and cucumber in a colandar and sprinkle with ½ teaspoon of kosher salt and let the excess liquid drain for two hours. (this step is optional, but recommended)
Prepare the dressing by first cutting the jalapeno into quarters by slicing in half lengthwise, and slicing each half lengthwise again. Leave the seeds and membrane attached.
Combine vinegar, sour cream, dill, sugar, black pepper, crushed red peppers, and jalapeno spears in a small bowl and mix to combine.
Set the dressing in the refrigerator to meld while the cucumber slices drain.
Finally, add the dressing to a large bowl with the sliced cucumbers and onions. Mix well to combine.*