4 pounds green cabbage, very thinly sliced on a mandoline or finely shredded
in a food processor
1 Granny Smith apple—peeled, cored and very thinly sliced on a mandoline
2 tablespoons plus 1 teaspoon fine sea salt
1 tablespoon caraway seeds
1/2 tablespoon juniper berries (optional)
Combine all of the ingredients in a very large bowl. Squeeze the cabbage to release some liquid. Press a heavy plate on the cabbage to weigh it down and let stand at room temperature, tossing and squeezing the cabbage 4 or 5 more times, until it has released enough liquid to cover, about 4 hours.
Pour the cabbage and its liquid into a clean ceramic crock or tall glass container. Top the cabbage with a clean plate that just fits inside the crock. Place a glass or ceramic bowl on the plate and put a heavy can in the bowl; the cabbage should be completely submerged in its brine by at least a 1/2 inch. Cover the crock with a clean kitchen towel and set it in a cool, dark place to ferment for about 6 weeks.
Every 3 days, clean and replace the plate that sits on the cabbage, carefully skimming any foam or mold that forms on the surface of the liquid. Discard the cabbage and its liquid if it's foul-smelling, or if anything brown, moldy or slimy has penetrated below what can easily be scraped off the surface. If too much liquid evaporates before the sauerkraut is sufficiently fermented, dissolve 1/2 teaspoon of sea salt in 1 cup of spring water and add it to the crock. When the sauerkraut is ready, it should have a light crunch and a bright, pleasantly tangy taste, with an acidity similar to that of a lemon.