Preheat oven to 350 degrees F. Grease a 9x13-inch or similar casserole dish.

In a large skillet melt 2 tablespoons of the butter over medium heat. Add mushrooms and season with salt and pepper. Cook until softened, about 5 minutes. Add spinach and garlic and continue to cook another 1-2 minutes, stirring frequently, until spinach is wilted. Transfer mixture to a bowl and add cooked chicken.

Return skillet to heat and melt the final 2 tablespoons of butter. Add flour and stir until combined. Pour in milk and whisk constantly to incorporate. Bring to a boil and cook until thickened.

Reduce heat to low and stir in ricotta cheese, 1/2 cup of parmesan cheese, a heavy pinch of salt and pepper, and nutmeg. Stir until smooth.

Spread a spoonful of sauce onto bottom of casserole dish. Cover bottom with lasagna noodles, gently breaking them to fit as necessary. Spoon more sauce over noodles, followed by 1/3 of chicken mushroom mixture, and 1/4 of of mozzarella cheese. Repeat two more times. Top final layer with the rest of the sauce, mozzarella cheese, and final 1/2 cup of parmesan cheese.

Cover with foil and bake for 35 minutes. Remove foil and bake uncovered for 10-15 minutes until cheese is lightly browned.

Let sit for about 15 minutes, then serve and enjoy!