Preheat the oven to 400°F. Spray an 11x7-inch baking dish with nonstick cooking spray, set aside.
In a large stockpot, over high heat, bring 4 cups of water to a boil.
Slice the green peppers in half lengthwise and remove ribs and seeds. Place the pepper halves, two at a time, in boiling water for 2 to 4 minutes. Repeat with the remaining peppers. (The pepper “skin” will lose a little brightness after cooking in boiling water.)
Use tongs to carefully remove the peppers from water and place open side down on paper towels to drain excess water.
Heat the olive oil in a large skillet over medium-high heat. Add the mushrooms, red bell pepper, onion, garlic, salt, and black pepper. Sauté for 2 to 3 minutes until the onion and peppers have softened.
Add the thinly sliced flank steak to the skillet. Stir in the Worcestershire sauce and cook for 2 to 4 minutes. Flank steak should still be slightly pink in the middle or at an internal temperature of 120º F for medium-rare , 130º F for medium.
Place the green bell pepper halves open side up in baking dish. Line the inside of each pepper with 7.5g of shredded cheese. Using a slotted spoon, transfer ½ cup of the flank steak mixture to each bell pepper half. Top each pepper with the remaining 7.5g of shredded cheese.
Bake the peppers for 10 minutes in preheated oven until cheese is melted.