Preheat oven to 350 degrees F. Spray a 2-quart baking dish with cooking spray and set aside.
In a large skillet, cook beef, onion and garlic over medium heat until no longer pink; drain.
Stir in the spinach, cream cheese, condensed soup, sour cream, milk, salt and 1 cup of the mozzarella cheese. Stir over low heat, just until combined.
Transfer to the prepared baking dish. Cover with foil. Bake for 20 minutes. Remove the foil; sprinkle with the remaining 1 cup of mozzarella cheese and the French-fried onions. Bake for 10 more minutes, or until the cheese is melted and the casserole is hot and bubbly.
