Preheat your oven to 375°F (190°C) and arrange the bell pepper halves in a baking dish.
In a large skillet, cook the ground beef over medium heat until browned; drain excess fat.
Add the cooked spinach and diced tomatoes to the beef; stir in oregano, garlic powder, salt, and pepper.
Spoon the beef mixture into each bell pepper half, pressing down lightly to fill.
Sprinkle the shredded mozzarella cheese evenly over the stuffed peppers.
Bake in preheated oven for 25-30 minutes until the peppers are tender and the cheese is golden.