For an extra layer of flavor, consider adding a sprinkle of grated Parmesan cheese to the stuffing topping before baking. If you don't have cream of celery soup, cream of mushroom or cream of chicken soup work wonderfully as substitutes. Ensure the spinach is thoroughly squeezed dry to prevent a watery casserole. Feel free to adjust the amount of stuffing to your preference. A thicker layer will create a more substantial crust. This casserole can be assembled ahead of time and refrigerated for up to 24 hours before baking. Add 10-15 minutes to the baking time if baking from cold.