Pull 3 outer leaves off the head of cabbage and set aside. Shred the remaining cabbage in a food processor, mandolin or chop with a knife. Add it to a big bowl.
Sprinkle the cabbage with sea salt and mix and massage the sea salt into the cabbage for about 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.
Meanwhile, remove the seeds from the jalapeños for a medium heat or leave them in for a spicier kraut and add them to a food processor. Be careful when handling the peppers, because they can burn and irritate the skin and eyes. I suggest wearing gloves when handling them and wash hands immediately after.
Add the garlic, cilantro and onion to the food processor too and process until the ingredients are finely minced. If you don’t have a food processor, finely shop with a knife. Set aside.
Go back to the cabbage and mix and squeeze the cabbage with your hands again. If liquid comes out when squeezed then it is ready for the next step.
Add the jalapeño mixture to the cabbage and mix it all together. Use your hands only if you wear gloves, otherwise use a spoon.
Pack the cabbage in the mason jar tightly with either a gloved hand or a vegetable pounder. I used the pounder that came with my Vitamix and that worked perfectly. Push it all the way down until it submerges in its own juices (this is the brine).
Repeat this until there is about 1½ inches of space from the top of the jar. Roll up the leaves and place them in the jar to push the cabbage under the brine.
Screw on the jar loosely so gas can escape as fermentation takes place. 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 a little and become cloudy. If scum appears, remove it with a spoon.
Remove the rolled up cabbage leaves and toss in the garbage before eating. Store sauerkraut in the fridge.