Preheat Oven: Preheat your oven to 375 degrees F (190 degrees C).
Prepare Bread: Slice the loaf of bread in half lengthwise. Spread mayonnaise over both halves of the bread and place them on a baking sheet.
Sauté Vegetables: In a large skillet, heat the olive oil over medium heat. Add the diced onion, green bell pepper, and mushrooms. Sauté until the vegetables are tender and the onions are translucent, about 5-7 minutes.
Cook Beef: Add the ground beef to the skillet and cook until browned, breaking it up with a spoon as it cooks. Drain any excess fat.
Season Beef: Season the beef mixture with Worcestershire sauce, salt, and black pepper.
Thicken Sauce: In a small bowl, whisk together the cornstarch and beef broth until smooth. Pour this mixture into the skillet and simmer until the sauce has slightly thickened, about 2-3 minutes.
Assemble Bread: Spread the beef and vegetable mixture evenly over both halves of the bread. Top with shredded cheese.
Bake: Bake in the preheated oven for about 15 minutes, or until the cheese is melted and bubbly.
Serve: Remove from the oven, let it cool slightly, then cut into portions. Sprinkle with red pepper flakes before serving.