Preheat the oven to 425°F (220°C). Line two baking sheets with aluminum foil and spray the foil generously with nonstick cooking spray.
In a small bowl, combine the whole milk and mashed saltine crackers. Stir and let soak until the crumbs are very soggy and have absorbed the milk (texture like over‑softened cereal).
In a large bowl, add the ground beef and ground sausage, breaking the meat into small pieces. Add minced garlic, eggs, Italian seasoning, kosher salt, black pepper, grated Parmesan, and finely chopped parsley, but do not mix yet.
When the saltine mixture is thoroughly soaked, add it to the meat bowl. Using your hands, gently but thoroughly combine until evenly mixed; stop as soon as ingredients are uniform to avoid overworking the meat.
Shape the mixture into meatballs about the size of a golf ball (approximately 1 to 1½ inch scoop). Place meatballs on the prepared baking sheets, spacing them evenly and keeping no more than 12 per sheet.
Bake at 425°F (220°C) for 15–20 minutes, until browned and cooked through. Remove meatballs gently from the pan.
Transfer meatballs into a rich tomato sauce and simmer gently (barely bubbling) for several hours, stirring occasionally and scraping the bottom of the pan as you stir.
Serve meatballs with spaghetti when sauce and meatballs are ready.