In a large bowl combine salt, onion powder, pepper, garlic powder, oregano, Parmesan, parsley, milk, eggs, and onion. Mix well. Add the breadcrumbs and mix again.
Add the beef and sausage to the seasonings and mix to combine. (I find mixing by hand is most efficient-in which case gloves would be helpful).
Separate the meat mixture into 16-20 meatballs. Add a cube of cheese to the center of each meatball and make sure to bury it on all sides.
Optional: since this recipe makes so many meatballs, at this point you could take half of the meatballs and freeze them in a Ziplock bag for a crockpot meal on another day. Otherwise proceed with all 16-20 of the meatballs by emptying a jar of marinara sauce into a crockpot, then adding the meatballs, with another jar of marinara sauce separating the layers of meatballs with a little extra marinara on top.
Cook on high for 3 hours or on low for 5-6 hours.
Serve as meatball subs, or with pasta. These meatballs are delish!