While the peppers are roasting, prepare the Spanish rice according to package directions. If you are using another grain, I recommend substituting the liquid with chicken broth and adding a 14 oz. can of diced tomatoes for extra flavor.
While the grains are cooking, prepare the meat filling by combining the ground beef, ground pork, garlic powder, and dried oregano. Mix well until fully combined.
In a large skillet, brown the meat thoroughly over medium heat, breaking up large pieces as you cook. Drain any excess fat.
In a large bowl, combine the meat mixture with about half the cooked rice. Add more if necessary so that you have enough to fill all of your peppers. Reserve the extra for leftovers.
Use a large spoon to scoop the mixture into the peppers. Don't be afraid to heap it on! Top with a layer of marinara sauce and shredded mozzarella cheese.
Cover with foil and allow bake in the oven at 375°F for about 30-45 minutes. This will allow the rest of the moisture to steam and finish cooking the peppers.
Remove the foil and continue cooking until the cheese is melty and slightly browned, about 10 minutes.
Remove from oven and allow to rest for about 10 minutes. This will give them time to firm up, making them easier to handle.
Garnish with more cheese and fresh parsley. Serve warm.