Preheat and Prepare : Preheat your oven to 350°F (175°C). Lightly grease a medium-sized casserole dish and set it aside for later use.

Sauté the Aromatics : In a large skillet, melt the butter over medium heat. Add the chopped onion and cook for about 3-4 minutes until softened. Incorporate the minced garlic and sauté for an additional minute until it becomes fragrant. Then, add a pinch of nutmeg along with the cayenne pepper to introduce a subtle kick of heat.

Create the Creamy Base : Pour in the heavy cream, stirring to combine with the onion and garlic. Reduce the heat and incorporate the cream cheese, stirring continuously until it melts and achieves a smooth consistency.

Add the Spinach and Cheese : Stir in the thawed and drained spinach, mixing well to coat it evenly with the creamy sauce. Season with salt and pepper to taste. Add ⅓ cup of shredded Parmesan cheese, stirring until well combined.

Transfer and Top : Pour the spinach mixture into the prepared casserole dish, spreading it out evenly. Top with the remaining Parmesan cheese and breadcrumbs, creating a layer that will develop a golden, crispy finish when baked.

Bake : Bake in the preheated oven for 25-30 minutes, or until the top is bubbly and lightly golden brown. For a little extra browning, you can broil the casserole for an additional 2-3 minutes at the end of the cooking time.

Serve and Enjoy : Remove from the oven and let it cool for a few minutes before serving. This creamy, cheesy spinach casserole pairs well with roasted meats, poultry, or as part of a vegetarian spread.