In a large skillet, melt the butter over medium heat.
Add the chopped onion and minced garlic, and sauté until they are soft and fragrant, about 3-4 minutes.
Add the fresh spinach to the skillet in batches, allowing each batch to wilt before adding more. Cook until all the spinach is wilted, about 5-7 minutes.
Remove the spinach mixture from the skillet and set aside.
Make the Cream Sauce:
In the same skillet, add the flour and stir constantly for about 1-2 minutes to cook out the raw flour taste.
Gradually whisk in the heavy cream and whole milk, continuing to whisk until the mixture is smooth and thickened, about 3-5 minutes.
Add the ground nutmeg, salt, and black pepper to the sauce. Stir to combine.
Combine and Assemble:
Preheat your oven to 350°F (175°C).
Return the cooked spinach to the skillet with the cream sauce, and stir to combine.
Transfer the creamed spinach mixture to a 9x13 inch baking dish.
In a small bowl, combine the grated Parmesan cheese, shredded mozzarella cheese, and breadcrumbs.
Sprinkle the cheese and breadcrumb mixture evenly over the top of the spinach.
Bake the Casserole:
Place the baking dish in the preheated oven and bake for 20-25 minutes, or until the top is golden brown and crispy.
Remove the casserole from the oven and let it cool slightly before serving.
Enjoy this Creamed Spinach Casserole for a delicious and comforting side dish that’s perfect for any occasion!