Pull 3 outer leaves off the head of cabbage and set aside. Shred the remaining cabbage in a food processor or mandolin or chop with a knife.
Place shredded or chopped cabbage in a big bowl with the grated carrots, radishes and sea salt.
Massage the cabbage mixture with your hands until it breaks down and becomes soft (about 10 min).
Pack the cabbage in the mason jar tightly, pushing it all the way down until it submerges in its own juices (this is the brine). Leave about 1½ inches of space from the top of the jar. If there is not enough brine to cover the cabbage, add more brine by combining a teaspoon of sea salt with 1 cup of water.
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 7 days in a cool, shaded place. 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.
Notes
Sauerkraut will keep in the refrigerator for a few months.
Recipe by Fermented Food Lab at https://www.fermentedfoodlab.com/carrot-radish-sauerkraut/