Step 1: Prepare the White Sauce
Melt butter in a saucepan over medium heat. Add the flour and whisk continuously for 1-2 minutes to form a roux.
Gradually pour in the milk while whisking to prevent lumps. Cook until the sauce thickens, about 4-5 minutes.
Add garlic powder, onion powder, nutmeg (if using), salt, and pepper. Stir until well combined, and set aside.
Step 2: Prepare the Chicken and Spinach
In a large skillet, sauté the spinach until wilted. Season with a pinch of salt and set aside.
If using raw chicken, cook it in the same skillet with a little olive oil until fully cooked, then shred or dice it.
Step 3: Assemble the Lasagna
Preheat the oven to 375°F (190°C).
Spread a thin layer of white sauce on the bottom of a greased 9x13-inch baking dish.
Place three lasagna noodles on top of the sauce.
Add a layer of chicken, followed by sautéed spinach, and a layer of white sauce. Sprinkle mozzarella and Parmesan cheese on top.
Repeat the layers two more times, ending with a generous layer of cheese on top.
Step 4: Bake the Lasagna
Cover the baking dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 15 minutes, or until the cheese is golden and bubbly.
Let the lasagna cool for 10 minutes before serving.