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.
Notes
Swap out the ground beef and use sausage or ground turkey instead.
Add extra vegetables to the casserole. Sautéed mushrooms would be great, and so would corn, canned (drained) diced tomatoes, or even diced and peeled boiled eggplant.
Instead of cream of mushroom soup, try cream of celery soup or cream of chicken soup for a slightly different flavor.
Make sure that you squeeze the spinach very dry before adding it to the casserole. I like to use a dish towel to wring out the liquid, which prevents a watery, soggy casserole.
Start with softened, room temperature cream cheese, sour cream and milk. This will help the ingredients come together smoothly.