In a large lidded jar or 3 smaller (16-ounce or similar) jars, place the cabbage, onion, and garlic.
In a small pot over medium heat, combine the apple cider vinegar, water, sugar, salt, and cumin seeds, if using. Simmer until the sugar and salt dissolve, about 1 minute. Pour the brine over the cabbage mixture in the jar(s).
Cover and shake gently to submerge the cabbage in the brine. Uncover and let cool to room temperature, pressing the cabbage down if needed to fully submerge it. Cover and chill for at least 1 hour or overnight.
