01 - Preheat oven to 375°F. Line a baking dish with parchment paper or lightly grease it.
02 - Place bell pepper halves cut side up in the prepared baking dish.
03 - Heat olive oil in a large skillet over medium-high heat. Add onions and cook until softened, about 2 to 3 minutes.
04 - Add mushrooms and diced green bell pepper if using. Sauté for 3 to 4 minutes until softened.
05 - Stir in minced garlic and cook for 30 seconds until fragrant.
06 - Add thinly sliced beef, Worcestershire sauce, salt, and black pepper. Sauté 3 to 5 minutes, stirring frequently, until beef is browned and fully cooked. Remove from heat.
07 - Place one slice of cheese inside each bell pepper half.
08 - Divide beef mixture evenly among the pepper halves, pressing gently to fill.
09 - Place a second slice of cheese on top of each stuffed pepper.
10 - Cover the baking dish with foil and bake for 25 minutes.
11 - Remove foil and bake an additional 5 to 7 minutes until cheese bubbles and turns golden.
12 - Allow peppers to cool slightly before serving.