Pull 3 outer leaves off the head of cabbage, set aside and cut cabbage in half.
Remove the cores from the cabbage and set aside.
Cut the cabbage into thin strips and place in a large bowl. You can also use a food processor or cabbage shredder for this step.
Add the salt to the cabbage. Using your hands, mix and massage (squeeze) the salt into the cabbage for 5 minutes. Set the cabbage aside and let it sit for 15-20 minutes so the sea salt has time to draw out the liquid and make the cabbage soft.
Mix and squeeze the cabbage until it's soft and when squeezed juice comes out. If you want to add other veggies, spices or herbs to your kraut, do it in this step. Mix and squeeze them with the cabbage.
Pack the cabbage in the mason jar tightly with either your hand or a vegetable pounder. Push it all the way down until it submerges in its own juices (this is the brine).
Fill the jar until there is about 1-2 inches of space from the top. Pour the rest of the brine into the jar to cover the cabbage in brine.
Take one of the cabbage cores and place it on top of the cabbage. This will act as a weight to keep the cabbage submerged under the brine. Or use a fermentation weight to keep it submerged.
Roll up one of the outer cabbage leaves you set aside in step 1. Stuff the rolled up cabbage leaf on top of the cabbage core. This will keep the sauerkraut under the brine.
Screw on the jar loosely so gas can escape as fermentation takes place Or use and airlock lid. Set on the counter for 5-7 days in a cool, shaded place.
Place a plate under the mason jar in case it bubbles over and makes a mess.
During fermentation the sauerkraut will bubble, become dull in color and the brine will get cloudy. When ready, store the sauerkraut in the refrigerator. Remove the rolled up cabbage leaves and toss in the garbage before eating. Sauerkraut will last for several months.
Recipe by Fermented Food Lab at https://www.fermentedfoodlab.com/how-to-make-sauerkraut/