1. In a 4-quart electric slow cooker, combine 2/3 of the mushrooms and the onion. In a small bowl, whisk together the undiluted soup with the wine, 1 tablespoon of the Worcestershire sauce, and the chiles. Pour half of the soup mixture over the mushrooms and onion in the slow cooker. Cut the beef horizontally into 2 pieces, each about 3/4 inch thick. Place on top of the mushroom mixture. Scatter the remaining mushroom slices over the meat and pour the remaining soup mixture evenly over all. Do not mix.
2. Cover and cook on the low heat setting about 8 hours, or until the beef is tender but not falling apart. Increase the heat setting to high
3. Mix the remaining 1 tablespoon Worcestershire sauce with the flour and several spoonfuls of the liquid from the slow cooker until smooth. Stir the flour mixture into the sauce in the slow cooker. Cover and cook on high 1/2 hour, or until slightly thickened.