Pick a glass container that has a tight fitting lid. I prefer the tall skinny canning jars that are used for asparagus.

Slice the onion in half, pole to pole. Cut the top and bottom ends off and peel the onion. Take 1/2 the onion, flat side down and slice lengthwise.  Repeat with the 2nd half. How thick you slice is completely your preference. I prefer more on the thin side.

Place onion slices in a medium bowl, sprinkle salt over top and lightly stir. Let marinate for 30 minutes until the onion releases some of its liquid. The onion will also soften slightly.

Transfer onions with salt and released liquid to your glass jar. Add the peppercorns, oregano, cumin, garlic, bay leaf and jalapeno (if using). Pour vinegar over top, add enough to cover onions completely. Place lid on jar and shake contents to distribute the seasonings.

Keep covered and let sit at room temperature to marry the flavors before you  use; I recommend at least 2 hours.

Store, covered in the refrigerator. Good for 3-4 weeks.