Preheat the oven to 425°F (218°C). Line two baking sheets with aluminum foil and spray the foil generously with nonstick cooking spray; set sheets aside.
In a small bowl, combine the crushed saltine crackers and whole milk. Stir frequently until the crackers have absorbed the milk and become very soggy, about a few minutes.
In a large bowl, add ground beef, ground sausage, minced garlic, eggs, Italian seasoning, kosher salt, black pepper, grated Parmesan, and chopped parsley. Tear the meat into small pieces while adding it to the bowl.
Add the milk-and-cracker mixture to the meat bowl, then gently mix with your hands until evenly combined; stop as soon as ingredients are incorporated to avoid overworking the meat.
Portion the mixture into balls about the size of a golf ball (a 1 to 1½ inch scoop works well). Place meatballs on the prepared baking sheets, no more than 12 per sheet.
Bake the meatballs at 425°F for 15–20 minutes, until cooked through and browned on the outside.
Carefully remove the meatballs from the pans and gently transfer them into simmering tomato sauce. Simmer gently (barely bubbling) for several hours, stirring occasionally and scraping the bottom of the pot as you stir.
Serve the meatballs over cooked spaghetti when the sauce and meatballs are ready.
