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!
NOTES:
- You can used frozen spinach. Thaw and squeeze out all excess moisture.
- I like to use no-boil, oven-ready lasagna noodles to cut down on prep work, but you can also use traditional lasagna noodles. Prepare according to package instructions.
- This lasagna can be made ahead: Assemble and cover and chill in the fridge for up to 24 hours before baking. Or assemble, double-wrap and freeze for a month or more. Bake from frozen, adding approximately 15 minutes to baking time.
Calories: 499kcal, Carbohydrates: 31g, Protein: 33g, Fat: 27g, Saturated Fat: 16g, Cholesterol: 105mg, Sodium: 842mg, Potassium: 561mg, Fiber: 2g, Sugar: 5g, Vitamin A: 3472IU, Vitamin C: 9mg, Calcium: 511mg, Iron: 2mg