Set a slow cooker to high heat. Place the onions and mushrooms in the bottom of the slow cooker.
Mix the remaining ingredients together in a small bowl. Pour evenly over the onions and mushrooms and toss to thoroughly coat.
Slow cook for 4 hours, until the onions and mushrooms are soft and dark.
Turn off the heat, and sprinkle the arrowroot evenly on top. Stir in the arrowroot, and let the onions and mushrooms sit for 10 minutes. Serve immediately.
Refrigerate any leftovers in an airtight container for up to five days.