Combine all ingredients--except the carrots, onions and arrowroot--and marinate the pot roast for 3 or 4 days. When it is about cooking time, presoak the pot, top and bottom, in water for 15 minutes. Place the roast in the pot along with the marinade, carrots and onions. Place the covered pot in a cold oven. Set temperature to 480 degrees. Cook 70 minutes (for a medium roast). Remove pot from oven and pour off liquid into a saucepan. Add the arrowroot and bring to a boil until thickened. Serve the sauce over brown rice.