
Layer the potatoes, carrots, onion and celery in a 5-quart slow cooker.
Place the flour in a large shallow dish. Add the stew meat, and turn to coat each piece evenly. In a large skillet, brown the meat in oil in batches. Transfer the browned meat to the slow cooker, placing it over the vegetables. Editor's Tip: When browning the meat, make sure there is space between each piece of beef. Overcrowding the skillet will cause the meat to steam instead of brown, so it won't take on a caramelized flavor.
In a large bowl, combine the tomatoes, broth, mustard, salt, pepper, thyme and, if desired, the browning sauce. Pour the stewing liquid over the beef. Editor's Tip: For added depth, pour the stewing liquid into the skillet and scrape the bottom of the skillet to pull up any browned bits from the beef. Then, transfer the liquid to the slow cooker.
Cover and cook the stew on low for seven to eight hours, or until the meat and vegetables are tender. If desired, sprinkle with fresh thyme before serving.