Preheat Oven to 350F.
Mix stuffing mix with hot water in a large bowl and set aside.
Heat olive oil in a skillet. Add chopped onions (sauté until translucent), add minced garlic, and chopped mushrooms sauté until tender, about 8 minutes.
Add the mushroom mixture to the prepared stuffing along with the spinach and cheese; mix well.
Pour into a greased baking dish and bake for about 30 minutes or until as crispy as you like. Add the optional 1/2 cup of shredded mozzarella cheese to the top if you like just before baking.