1. Prepare the Base Ingredients:
* Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish (or a similar 2.5-3 quart casserole dish).
* If you haven’t already, thaw the frozen spinach. Place the thawed spinach in a clean kitchen towel or several layers of robust paper towels and squeeze out every bit of excess liquid. This is a critical step! Set the dry spinach aside.
* Drain, rinse, and roughly chop the artichoke hearts. Shred or dice your cooked chicken.

2. Sauté Aromatics and Vegetables:
* In a large skillet or Dutch oven, heat the olive oil over medium heat.
* Add the chopped onion and cook for 5-7 minutes, stirring occasionally, until softened and translucent.
* Add the minced garlic and cook for another 1-2 minutes until fragrant. Be careful not to burn the garlic.
* Add the squeezed-dry spinach and chopped artichoke hearts to the skillet. Cook for 3-5 minutes, stirring, to allow any remaining moisture to evaporate and the flavors to meld.
* Remove the skillet from the heat. Stir in the cooked chicken. Season this mixture lightly with salt and pepper. Set aside.

3. Make the Creamy Cheese Sauce:
* In a medium saucepan, melt the butter over medium heat.
* Once the butter is melted, whisk in the all-purpose flour. Cook for 1-2 minutes, whisking constantly, to create a smooth roux. This step cooks out the raw flour taste.
* Gradually whisk in the milk and chicken broth. Add the liquid slowly at first, whisking vigorously to prevent lumps. Once all the liquid is incorporated, continue to cook, whisking frequently, until the sauce comes to a gentle simmer and begins to thicken (about 5-7 minutes).
* Reduce the heat to low. Add the cubed softened cream cheese, whisking until it’s fully melted and incorporated into the sauce.
* Stir in the 1/2 cup of grated Parmesan cheese, salt, pepper, pinch of nutmeg (if using), and red pepper flakes (if using). Continue to stir until the Parmesan is melted and the sauce is smooth and creamy.
* Taste the sauce and adjust seasonings if necessary. Remember that the chicken mixture is also seasoned.

4. Combine and Assemble the Bake:
* Pour the creamy cheese sauce over the chicken, spinach, and artichoke mixture in the large skillet or Dutch oven (or transfer the chicken mixture to a large bowl and pour the sauce over it).
* Stir everything together gently but thoroughly until all the ingredients are well coated with the sauce.
* Pour the entire mixture into your prepared 9×13 inch baking dish, spreading it into an even layer.

5. Top and Bake:
* Sprinkle the 1.5 cups of shredded mozzarella cheese evenly over the top of the chicken mixture.
* Sprinkle the 1/4 cup of grated Parmesan cheese over the mozzarella.
* If using, sprinkle the breadcrumbs (plain or Panko) evenly over the cheese for an extra crunchy topping.
* Place the baking dish in the preheated oven. Bake for 25-35 minutes, or until the casserole is hot and bubbly around the edges, and the cheese topping is melted, golden brown, and slightly crispy in spots.

6. Rest and Serve:
* Once baked, carefully remove the Chicken Florentine Artichoke Bake from the oven.
* Let it rest for at least 10-15 minutes before serving. This allows the sauce to set slightly and makes it easier to serve without it being too runny. It also prevents burning mouths!
* Garnish with fresh chopped parsley, if desired, just before serving.

Serve warm and enjoy the creamy, cheesy, flavorful goodness!