Follow these detailed steps carefully to ensure your stuffed peppers are cooked to perfection, with a tender pepper shell and a hot, bubbly, and flavorful filling.

1. Prepare the Bell Peppers: Begin by preheating your oven to 375°F (190°C). Wash the bell peppers thoroughly. Slice each pepper in half lengthwise, from the stem to the base. Carefully remove the seeds and the white membranes from the inside. For a more stable base, you can slice a very thin sliver off the rounded bottom of each pepper half, being careful not to create a hole.
2. Par-Boil the Peppers (Optional but Recommended): Bring a large pot of salted water to a rolling boil. Place the pepper halves in the boiling water and cook for 3-4 minutes. This step, known as blanching, helps to soften the peppers and ensures they will be perfectly tender after baking, rather than unpleasantly crunchy. Immediately remove them with tongs and place them in an ice bath or run them under cold water to stop the cooking process. Pat them dry and arrange them cut-side-up in a 9×13-inch baking dish.
3. Cook the Aromatics and Beef: In a large skillet or Dutch oven, heat the olive oil over medium-high heat. Add the finely chopped onion and cook until it becomes soft and translucent, about 5-6 minutes. Add the minced garlic and cook for another minute until fragrant, being careful not to let it burn.
4. Brown the Filling: Add the ground beef to the skillet with the onions and garlic. Use a wooden spoon or spatula to break the beef apart. Cook until it is thoroughly browned and no pink remains, which should take about 7-8 minutes. Drain off any excess grease from the skillet.
5. Combine the Filling Ingredients: Reduce the heat to medium. Stir in the drained diced tomatoes, tomato sauce, cooked rice, dried oregano, dried basil, and smoked paprika. Season generously with salt and black pepper. Mix everything together until it is well combined. Let the mixture simmer for about 5 minutes, allowing the flavors to meld together beautifully.
6. Incorporate the Cheese: Remove the skillet from the heat. Stir in ½ cup of the shredded cheddar cheese and ½ cup of the shredded mozzarella cheese. Mixing cheese directly into the filling is the key to an ultra-cheesy experience in every single bite.
7. Stuff the Peppers: Carefully spoon the beef and rice mixture evenly into each of the prepared bell pepper halves. Mound the filling generously, as it will settle slightly during baking.
8. Prepare for Baking: Pour about ½ cup of water or beef broth into the bottom of the baking dish, around the peppers. This creates steam in the oven, which helps keep the peppers moist and cook them evenly.
9. Add the Final Cheese Topping: In a small bowl, combine the remaining 1 cup of cheddar cheese and ½ cup of mozzarella cheese. Sprinkle this cheese mixture generously over the top of each stuffed pepper, ensuring the filling is completely covered.
10. Bake to Perfection: Cover the baking dish loosely with aluminum foil. This prevents the cheese from browning too quickly before the filling is heated through. Bake in the preheated oven for 25 minutes.
11. Final Browning: After 25 minutes, carefully remove the aluminum foil. Return the dish to the oven and bake for another 10-15 minutes, or until the filling is heated through and the cheese on top is completely melted, bubbly, and has developed beautiful golden-brown spots.
12. Rest and Garnish: Remove the baking dish from the oven and let the stuffed peppers rest for at least 5-10 minutes. This allows the filling to set, making them easier to serve and preventing them from falling apart. Just before serving, sprinkle with freshly chopped parsley for a burst of color and freshness.