In a 9x13 baking dish, spread marinara sauce; set aside.
Preheat oven to 375ºF.
Prep the bell peppers and set the tops aside for other recipes. Cut the bottoms flat if the peppers are not sitting straight; set aside.
In a large saucepan over medium heat, add oil and when hot, add onions and celery; season with salt and sauté for 4 minutes or until they start to soften. Add garlic and sauté for 30 seconds.
Transfer the onion mixture to a large bowl; rest for 5 minutes. Add ground beef, hot Italian sausage, diced tomatoes, cooked rice, 1 cup Parmigiano-Reggiano cheese, basil leaves, oregano, parsley, 2 generous pinches salt and freshly ground black pepper; mix the ingredients very well. To test the seasoning, form a small patty and cook; adjust if necessary.
Stuff the peppers and place them in the prepared baking dish.
In a small bowl, add the remaining ¾ cup Parmigiano-Reggiano cheese and Mozzarella cheese; stir well. Spread topping over the stuffed peppers and sprinkle with paprika.
Lightly grease some foil and cover the peppers, forming a tent to prevent the foil from touching the cheese topping.
Transfer to the preheated oven and bake for 45 minutes.
Remove the foil and bake for an additional 30 minutes or until the peppers are soft (test with a pointy knife) and the cheese is melted.
Remove from the heat and let them rest for 10 minutes before serving; sprinkle parsley on top.