1. Peel a few of the outer leaves from each head of cabbage and set aside.
2. Slice the rest of the cabbage as thinly as possible using a food processor, mandolin, or knife.
3. Place the sliced cabbage in a large bowl; if you don’t have a bowl big enough for all 3 pounds you can work in batches.
4. Sprinkle the cabbage with salt. Massage the cabbage with your hands to thoroughly distribute the salt and start the process of breaking down the cabbage.
5. Keep massaging until the cabbage is well wilted.
6. Make sure your fermentation crock or vessel is very clean. Pack the cabbage tightly into the vessel, handful by handful, pressing down firmly after each handful.
7. Place the large outer leaves from step 1 over the top of the sliced cabbage. (You may have to tear or fold the leaves to cover the entire surface.) This ensures that the sliced cabbage stays submerged under the brine.
8. Weigh down the cabbage. If you are using a fermentation crock, use the weight that comes with it.
9. Otherwise, a clean glass jar, slightly smaller in diameter than the mouth of your fermentation vessel, filled with water works well.
10. If using a fermentation crock, put on the lid.
11. Otherwise, cover the vessel with a breathable barrier (a paint-straining bag, nut mill bag, coffee filter, linen towel, several layers of cheesecloth, or even paper towels) and secure with a rubber band (unless you are using something like a paint-straining bag that has elastic around the opening).
12. Check the level of the liquid in the fermentation vessel over the next 24 hours.
13. If it is not at least 1 inch above the top of the cabbage, dissolve 1 teaspoon of salt into 1 cup of water (per batch).
14. Pour the salty water into the fermentation vessel until the liquid is a least 1 inch above the top of the cabbage.
15. The sauerkraut will be ready in as little as 5 days but can ferment for up to 5 months (check the liquid level periodically and top up with salted water as needed). A little bubbling or foaming is normal.
16. Once the sauerkraut is fermented to your liking, transfer to smaller jars (if desired) and store in the refrigerator for up to 6 months.