All you need is cabbage and salt, though additions like carrots, garlic, peppercorns, horseradish root, radishes, apples, or caraway seeds can add personality.
Start with a firm head of green cabbage. Remove the outer leaves and set aside. Slice the cabbage thinly (a mandolin helps—if you dare), discarding the tough core.
Place the cabbage (and any add-ins) in a bowl and weigh it, subtracting the bowl’s weight. Add 1.5–2% of that weight in non-iodized salt (sea, kosher, or Himalayan pink).
Let sit a few minutes, then massage with clean hands until the cabbage releases enough liquid to form a brine—this may take up to 10 minutes. Pack tightly into a sterilized jar, pressing out air pockets, and pour any remaining brine over the top.
Fold a reserved cabbage leaf over the surface to keep everything submerged. If needed, add extra brine (2 tablespoons salt per quart of non-chlorinated water).
Cover with an airlock lid, or a canning lid set loosely. Ferment at room temperature (70–75°), out of direct sunlight, for 5 to 7 days.
Keep everything submerged—anything exposed to air can mold. You’ll see bubbles as it ferments; start tasting around day five. When it’s pleasantly tangy, refrigerate to slow the process.