To assemble: Put a whole raw pot roast (4lb or so) in a Ziploc bag. On top of the roast, layer 1 cup of chopped onion, half a bag of baby carrots (or 3 large carrots, chopped), 3 stalks of chopped celery, and half a container of mushrooms. On top of the veggies, sprinkle a pack of Onion Soup Mix and add a can of Cream of Mushroom soup. Seal and freeze.
To cook: Take out the bag the night before and let thaw overnight. The next morning, pour the roast/veggies in the slow-cooker (it's okay if it's partially frozen). Cook on low for 8-10 hours. Season with salt to taste and serve over rice.