Combine all ingredients in a large, resealable plastic bag. Place bag in roaster and marinate for 24 to 48 hours.
Preheat oven to 300ºF.
Pour contents of bag into roaster, arranging carrots around the roast, and mushrooms on top.
Cover and cook for 3 hours. Check the roast after 2 hours. Test for doneness by inserting a skewer, if the the juices run clear, roast is ready.