Slow Cooker Philly Cheesesteak Peppers

Heat the oil in a large skillet over medium heat. Add the beef, and cook until browned. Add the mushrooms and onion; cook and stir until starting to become tender, about 5 minutes. In a slow cooker, combine the beef broth, salt, pepper, fish sauce, and red pepper flakes. Transfer the beef and vegetables to the slow cooker, and stir to blend. Cover, and cook on High for 3 to 4 hours, until beef is extremely tender.
Preheat the oven to 425 degrees F (220 degrees C). Drain the liquid from the slow cooker, and save for dipping. Cut the top of the Green Peppers off to shape like a bowl. Mix together the horseradish and mustard; spread onto the inside of the pepper. Bake the pepper for 15 minutes or until slightly soft. Remove from oven. Place slices of provolone cheese on insides of the peppers, then fill with the beef and vegetables. Place back in the oven until cheese is melted and serve.