Preheat your oven to 350°F (175°C).
In a large skillet over medium-high heat, cook the sliced beef until browned, approximately 5-7 minutes.
Add sliced onions and bell peppers to the skillet, cooking for an additional 5 minutes until softened.
Reduce heat and stir in cream cheese, Worcestershire sauce, garlic powder, salt, and pepper; mix until the cream cheese is completely melted.
Transfer the beef and veggie mixture into a greased baking dish.
Top the mixture with shredded mozzarella cheese evenly.
Bake in the preheated oven for 20-25 minutes, or until the cheese is bubbly and golden.
Allow the casserole to cool for 5 minutes before serving.