Place cabbage in a large non-reactive bowl or plastic food-safe bucket. Sprinkle salt over cabbage and toss to combine. Cover with plastic wrap or a lid. Let soak in the refrigerator, 8 hours to overnight.

Squeeze out liquid from the cabbage; drain.

Combine water, vinegar, sugar, oil, turmeric, celery seed, and mustard seeds in a large pot; bring to a boil. Stir in drained cabbage, onions, and red pepper. Bring back up to a boil. Reduce heat and simmer until onions are very tender, about 45 minutes.

Sterilize 6 quart jars and lids in boiling water for 5 minutes. Pack cabbage mixture into hot, sterilized jars, filling to within 1 inch of the top. Run a clean knife around the insides of the jars to remove any air bubbles. Top with lids and screw on rings.

Place a rack in the bottom of a large pot and fill halfway with water. Bring to a boil and lower jars 2 inches apart into the boiling water using a holder. Pour in more boiling water if necessary to cover jars by 1 inch. Bring the water to a rolling boil, cover the pot, and process for 25 minutes.

Remove the jars from the pot and place onto a cloth-covered or wood surface, several inches apart, until cool. Press the top of each lid with a finger, ensuring that the seal is tight (lid does not move up or down at all). Store in a cool, dark area.