8 T. butter
1 lb. white mushrooms, sliced
1 c. finely chopped yellow onion
3 T. minced garlic
1/2 c. flour
7 c. milk
2 tsp. kosher salt
1/2 tsp. black pepper
1/4 tsp. grated nutmeg
1 lb. spinach, washed and large stems removed
3 c. grated Parmesan
2 T. olive oil
2 lb. boneless, skinless chicken breast
1 T. Italian seasoning
1 lb. oven ready lasagna sheets
Melt the butter in a large saucepan over medium heat. Add the mushrooms and cook, stirring often until the mushrooms are browned and most of the liquid has evaporated. Add the onions and garlic to the pan and saute until onions are translucent. Add the flour and cook, stirring about 2 minutes. Whisking constantly, slowly add the milk and continue to cook, stirring until thickened, about 5 minutes. Add 1 1/2 tsp. of the salt, pepper, and nutmeg. Add the spinach in batches until wilted into the sauce. Stir in 1 1/2 c. of the Parmesan and cook the sauce for 2 minutes more.
Preheat oven to 375.
In a large saute pan heat the olive oil. Season the chicken with salt, pepper, and Italian seasoning. Brown the chicken on both sides. Place pan in the oven and cook until no longer pink in the middle. Chop into bite size pieces.
Brush a 13 X 9 baking dish with olive oil. Spread 1/2 c. of the sauce on the bottom of the dish.
Add a single layer of pasta sheets. Top with sauce, chicken and cheese, in that order. Repeat 2 more times, creating 3 layers of pasta. End with a layer of pasta covered with the remaining sauce. Sprinkle the last of the Parmesan on top.
Place the casserole on a parchment lined baking sheet. Bake, uncovered, until brown and bubbly, about 45 minutes. Remove from the oven and allow to cool for 15 minutes before cutting.