Mix cabbage, onion, sea salt, and garlic together in a bowl. Firmly pack mixture into a large, clean, food-grade plastic bucket. The cabbage will start to make its own brine as the salt starts to draw out the water of the cabbage.
Fill a large, clean, food-grade plastic bag with water and place over the salted cabbage mixture so none of the cabbage is exposed to air.
Allow cabbage to ferment in a cool, dry place, 1 to 4 weeks (depending on how tangy you like your sauerkraut). The temperature of the room you ferment the sauerkraut in should not rise above 70 degrees F (21 degrees C).