Coat a large, wide pot with olive oil and add the bacon. Render the bacon over medium heat. Add the red onions, season with salt, and sweat for 4 to 5 minutes. Add the carrots and stir to combine. Add the cabbage, cider vinegar, mustard seeds, thyme, and bay leaves. Season with salt and taste. This will taste very vinegary but the salt will help offset the sharpness of the vinegar.
Reduce to a simmer, cover, and cook until the cabbage is very soft, 15 to 20 minutes. Remove the lid, add the apple cider, and simmer until most of the vinegar has absorbed but the cabbage is still moist but not soupy, an additional 10 minutes. Serve right away or let it hang out in the refrigerator. It will probably be better in a couple of days. Garnish with chives.