Preheat your oven to 375°F (190°C). Lightly grease a 9x13 baking dish and spread about 1/2 cup of tomato sauce across the bottom in an even layer.
Slice the tops off each bell pepper, then remove all seeds and white membranes from inside. Stand the peppers upright in the prepared baking dish, trimming the base if needed so they sit flat.
Heat 2 tablespoons of olive oil in a large skillet over medium-high heat. Add the diced onion and cook for 4 to 5 minutes until softened, then add the minced garlic and cook for 1 more minute until fragrant.
Add the ground beef to the skillet, breaking it into small crumbles, and cook for 6 to 8 minutes until fully browned with no pink remaining. Drain off any excess fat.
Reduce heat to medium and stir in the diced tomatoes, about 3/4 cup of the tomato sauce, Worcestershire sauce, Italian seasoning, garlic powder, onion powder, smoked paprika, salt, and pepper. Simmer for 3 to 4 minutes until slightly thickened.
Remove the skillet from heat and stir in the cooked rice until fully combined. Taste the filling and adjust seasoning as needed.
Spoon the beef and rice filling firmly into each pepper, mounding it slightly above the rim. Spoon the remaining tomato sauce evenly over the tops of the filled peppers.
Sprinkle shredded mozzarella evenly over each filled pepper, then cover the baking dish tightly with aluminum foil.
Bake on the center rack at 375°F for 40 minutes until the peppers are tender and the filling is heated through.
Remove the foil and bake for an additional 12 to 15 minutes until the cheese is melted, golden, and bubbling. Check doneness by piercing a pepper with a knife; it should meet no resistance.
Let the stuffed peppers rest for 5 minutes before serving. Garnish with freshly chopped parsley and serve hot.