In a large bowl, combine ground beef or turkey, breadcrumbs, Parmesan cheese, parsley, egg, garlic powder, onion powder, salt, and black pepper.
Gently fold in the cubed mozzarella cheese, being careful not to overmix the meat mixture.
Form the mixture into 1.5-inch meatballs and place them on a plate or tray.
In a slow cooker, spread half of the marinara sauce on the bottom to prevent sticking.
Place the meatballs on top of the sauce and pour the remaining sauce over them, ensuring they're well-coated.
Sprinkle the Italian seasoning over the top of the sauce-covered meatballs.
Cover the slow cooker and cook on low for 6 to 8 hours or on high for 3 to 4 hours.
Once cooked, carefully serve meatballs with sauce over pasta or on sub rolls.
