Cover roast with water in crockpot and simmer overnight or approximately 8 hours. Let cool.
Take meat off bone and separate into smaller pieces. Add meat, tomatoes, chiles and diced onions to the broth. Add about 2 teaspoons salt and 1/2 teaspoon pepper. Let simmer another 8 hours. Thicken with flour and water.
Let set overnight in refrigerator. Serve the next day alone, with tortillas or over burritos.