Set your oven's broiler to preheat. In a separate small bowl, whisk together the beef broth and cornstarch until smooth, then set aside. In a large nonstick skillet over medium-high heat, warm the olive oil. Sauté the diced onion, bell pepper, and chopped mushrooms for approximately 3 minutes until tender. Add the ground beef to the skillet, breaking it apart as it cooks for another 3 minutes until browned, then drain any excess fat.

Incorporate the ketchup, Worcestershire sauce, taco seasoning, salt, and black pepper into the skillet and stir to combine. Re-stir the cornstarch and broth mixture and pour it into the skillet; let simmer for around 3 minutes, stirring intermittently, until the sauce thickens slightly.

Evenly distribute the beef mixture over the hamburger buns. Top each with about 1/4 cup of shredded cheese and place them under the broiler until the cheese is melted—monitor closely to prevent burning. Alternatively, stir the cheese directly into the meat mixture until melted and incorporated. Serve warm, ideally with chips or your preferred side.