Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish.

Melt 4 tablespoons of unsalted butter in a large skillet over medium heat, then add 1 finely chopped yellow onion and cook for 3-4 minutes until translucent. Add 2 cloves of minced garlic and sauté for an additional 30 seconds.

Sprinkle 1/4 cup of all-purpose flour over the onion and garlic mixture, stirring constantly for about 1 minute to form a roux.

Gradually whisk in 1 cup of whole milk and 1 cup of heavy cream, stirring for 4-5 minutes until the sauce thickens and bubbles gently.

Season with salt, ground black pepper, and ground nutmeg (if using). Lower the heat and mix in 2 cups of shredded sharp cheddar cheese and 1/2 cup of grated Parmesan until melted and smooth.

Fold in the well-drained spinach, ensuring it's evenly coated with the sauce.

Pour the spinach mixture into the prepared baking dish, spreading it evenly. Top with remaining grated Parmesan and 1 cup of seasoned breadcrumbs.

Bake uncovered for 25-30 minutes until golden brown and bubbling.

Let it rest for 5-10 minutes before serving.