While sauce is simmering, heat oven to 475 degrees (450 convection). Spray large rimmed baking sheet with cooking oil.
In bowl with preserved half of the onion mixture, add bread and milk and mash together until smooth. Add sausage, parsley, parmesan, eggs, garlic and salt to bowl and mash to combine. Add beef and knead with hands until well combined.
Lightly shape mixture into 2 inch round balls (about 12-15 in total), and place on rimmed baking sheet.
Bake meatballs until well browned, about 20 minutes.
Transfer meatballs to large pot with tomato sauce that has been simmering already 45 minutes, and simmer together and additional 15 minutes, gently stirring occassionally.
Serve with pasta of your choice (or freeze up to 1 month)
Enjoy!