Cut the 4 large peppers in half lengthwise. Remove the seeds and ribs and discard. Drizzle the peppers with ½ tablespoon olive oil and season with salt and pepper. Place them skin-side-down on a rimmed baking sheet and bake for 15 minutes.
Meanwhile, heat 1 tablespoon of olive oil in a large nonstick skillet over medium-low heat. Add the diced green bell pepper, mushrooms and onion. Cook for about 10 minutes, until the vegetables are softened and beginning to caramelize. Sprinkle generously with salt. Remove the veggies from the skillet.
Adjust heat to medium, and add the ground beef to the skillet. Break it up with a spatula and cook for 5 to 7 minutes, until no longer pink. Add the garlic and a sprinkle of salt; cook for one minute. Add the vegetables back to the pan, and stir into the beef.
Add the tomato paste, Worcestershire sauce and mustard. Stir to combine. Add the broth and stir until the beef and vegetables are well-coated with the sauce. Stir in the brown rice. Season to taste with salt and pepper.
Divide the meat mixture among the bell pepper halves. Top each bell pepper with a slice of the Provolone cheese. Bake 12 to 15 minutes, or until the cheese is golden brown and the bell peppers are tender. Sprinkle with parsley, and serve.
Notes
If preparing this recipe as gluten-free, just be sure to use GF versions of Worcestershire sauce, Dijon mustard and beef broth.