Prepare the Chicken & Spinach Filling:
* Thaw the frozen spinach completely. Once thawed, place it in a clean kitchen towel or several layers of paper towels and squeeze out as much liquid as humanly possible. This step is crucial to prevent a watery lasagna. Set aside.
* In a large skillet or Dutch oven, heat the olive oil over medium heat. Add the chopped onion and cook for 5-7 minutes, until softened and translucent.
* Add the minced garlic and cook for another 1-2 minutes until fragrant, being careful not to burn it.
* Add the squeezed dry spinach to the skillet. Cook for 2-3 minutes, stirring, to evaporate any remaining moisture and allow the flavours to meld.
* Remove the skillet from the heat. In a large bowl, combine the cooked chicken, the sautéed onion-spinach mixture, ricotta cheese, beaten egg, 1/2 cup Parmesan cheese, salt, pepper, and a pinch of nutmeg (if using). Mix thoroughly until well combined. Set aside.

2. Make the Creamy Béchamel 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 cooks out the raw flour taste).
* Gradually whisk in the warm milk, a little at a time, ensuring each addition is fully incorporated before adding the next to prevent lumps.
* Continue to cook over medium heat, whisking frequently, until the sauce comes to a gentle simmer and thickens enough to coat the back of a spoon (about 5-8 minutes).
* Remove the saucepan from the heat. Stir in the 1 cup of Parmesan cheese, salt, pepper, and ground nutmeg. Whisk until the cheese is melted and the sauce is smooth. Taste and adjust seasonings if necessary.

3. Assemble the Lasagna:
* Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish.
* First Layer (Sauce): Spread about 1 cup of the béchamel sauce evenly over the bottom of the prepared baking dish. This prevents the noodles from sticking and helps them cook.
* Second Layer (Noodles): Arrange a single layer of no-boil lasagna noodles over the sauce. You may need to overlap them slightly or break them to fit.
* Third Layer (Filling): Spread half of the chicken and spinach ricotta mixture evenly over the noodles.
* Fourth Layer (Cheese & Sauce): Sprinkle one-third of the shredded mozzarella cheese (about 2/3 cup) over the ricotta mixture. Then, ladle about 1.5 cups of the béchamel sauce evenly over the mozzarella.
* Repeat Layers:
* Add another layer of no-boil lasagna noodles.
* Spread the remaining chicken and spinach ricotta mixture.
* Sprinkle another one-third of the shredded mozzarella cheese.
* Ladle another 1.5 cups of the béchamel sauce.
* Final Noodle Layer: Add a final layer of no-boil lasagna noodles.
* Top Layer (Sauce & Cheese): Spread the remaining béchamel sauce over the top noodles, ensuring they are completely covered (this is important for no-boil noodles to cook properly). Sprinkle the remaining shredded mozzarella cheese and the 1/2 cup of grated Parmesan cheese evenly over the top.

4. Bake the Lasagna:
* Cover the baking dish loosely with aluminum foil (you can tent it slightly or spray the underside of the foil with cooking spray to prevent the cheese from sticking).
* Bake in the preheated oven for 30 minutes.
* Remove the foil and continue to bake for another 15-25 minutes, or until the top is golden brown, the sauce is bubbly around the edges, and the lasagna is heated through (an internal temperature of at least 165°F or 74°C).

5. Rest and Serve:
* Once baked, remove the lasagna from the oven and let it rest for at least 15-20 minutes before slicing and serving. This crucial resting period allows the layers to set, making it much easier to cut clean slices.
* Garnish with fresh chopped parsley, if desired, before serving.