Combine 3 cups of the water and the salt in a large mason jar (32-50 oz). Stir until salt dissolves.
Press the cabbage down into the water until it is fully immersed. Use a ziploc bag with water in it to act as a weight. Press the bag into the jar, pressing the cabbage down so all of it falls below the water line.
In order for the cabbage to begin fermenting, nothing can peek up above the water level. Seal the jar tightly and let sit at room temperature for 7 days. The color of the contents of the jar will change on about day 5 or 6. It will become a deep purple-magenta color.
Drain most of the liquids (not all!) and heat the red cabbage kraut as desired before serving. Store leftovers in an airtight container in the fridge.
Notes
During the fermentation process, occasionally rotate the jar to help air bubbles come to the surface. This will aid the process. Be sure not to shake the contents of the jar, as this may add air instead of getting rid of it.
Add seasonings to the jar along with the salt and water, if desired. Options include peppercorns or black pepper, red pepper flakes, coriander seeds, caraway seeds, juniper berries or other herbs, spices or seasonings that cater to your personal preferences.
Use either one large mason jar or a handful of smaller mason jars (dividing up the salted water mixture) to make small batches.
An entire head of red cabbage can be used to double the batch.
If you love how this recipe turns out, consider repeating this same process with different vegetables such as green beans, carrots or cucumbers.