Preheat your oven to 400°F (200°C).
In a large mixing bowl, combine the ground beef, spicy Italian sausage, egg, breadcrumbs, parsley, minced garlic, chopped onion, crushed red pepper flakes, smoked paprika, salt, and black pepper.
Use your hands to thoroughly mix all the ingredients until evenly combined.
Form the mixture into small balls, about 1-2 inches in diameter.
Take each meatball and create a small pocket to place a bit of shredded mozzarella cheese inside, then seal the meatball around the cheese.
Heat the olive oil in a large, oven-safe skillet over medium-high heat.
Add the meatballs to the skillet and cook, turning occasionally, until they are browned on all sides.
Pour the marinara sauce over the meatballs and transfer the skillet to the preheated oven.
Bake for about 15-20 minutes, or until the meatballs are cooked through.
Remove from the oven and let the meatballs rest for a few minutes before serving.
Garnish with fresh basil and serve hot.