Combine the sliced cabbage, grated carrots, sliced onions, minced garlic, and oregano in a large bowl.

Sprinkle the salt over the vegetable mixture and massage it in for about 5-10 minutes until the vegetables start to release their juices.

Pack the vegetable mixture tightly into a clean glass jar, pressing down firmly to ensure the vegetables are submerged in their own juices. If needed, add a bit of filtered water to cover the vegetables.

If using whey, pour it over the vegetables.

Cover the jar with a lid loosely to allow gases to escape and place it in a dark, room-temperature spot for 3-7 days, depending on your desired level of fermentation.

Check the curtido every day, pressing the vegetables down to keep them submerged. Taste it after 3 days and continue fermenting until it reaches your desired tanginess.

Once ready, secure the lid tightly and store the curtido in the refrigerator. It will continue to develop flavor but at a much slower rate.