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 1/3 cup whole milk and 1/2 cup crushed saltine crackers. Stir and let soak until very soggy and fully absorbed, about 5–10 minutes.
In a large bowl, add the ground beef, ground sausage, minced garlic, eggs, Italian seasoning, kosher salt, black pepper, grated Parmesan, and chopped parsley; tear the meat into smaller pieces as you add it.
When the cracker-and-milk mixture is fully soggy, add it to the meat mixture. Gently mix with your hands just until ingredients are evenly combined; avoid overmixing to prevent tough meatballs.
Form the mixture into golf-ball–sized meatballs (about 1 to 1½ inch or using a 1–1½ inch scoop). Place no more than 12 meatballs per baking sheet.
Bake at 425°F (220°C) for 15–20 minutes, until browned and cooked through. Remove gently from the pans.
Transfer the meatballs to a pot of simmering tomato sauce and simmer gently for several hours if desired, stirring occasionally and scraping the bottom to loosen browned bits; serve over spaghetti.