Bring the beef broth and water to a boil in a saucepan over high heat. Stir in the cranberry sauce until dissolved. Pour the sauce into a slow cooker set to High.
Meanwhile, season the beef roast with salt and pepper, then sprinkle evenly with the flour. Heat the vegetable oil in large skillet over medium heat. Cook the roast in the hot oil until brown on all sides, about 2 minutes per side. Transfer the roast to the slow cooker along with the chopped onion.
Cook until the roast easily pulls apart with a fork, about 4 hours.
