While peppers are roasting, prepare the filling. Add olive oil, onions, garlic, and beef broth to a large skillet over medium heat. Cook for 3–5 minutes or until onions are translucent. Stir in mushrooms, chopped roast beef, Worcestershire sauce, cream cheese, 4 American cheese slices, and black pepper. Stir until cheese is melted and combined. Stir in cooked brown rice. Add additional broth if mixture seems dry. Taste for seasoning and adjust salt and pepper if desired.