Place the roast in a Dutch oven. Sprinkle the onion soup mix on top; pour on the mushroom soup, spreading it around a little to cover. Put the lid on and bake at 250 ° F for about 6 hours. When it's done, remove any remaining fat and shred the meat using 2 large serving forks (the meat should be very tender and easy to shred). Serve on buns.
To use your slow cooker, set it on low and cook for 6-8 hours.