1 (8 ounce) package sliced fresh mushrooms
salt and ground black pepper to taste
2 tablespoons unsalted butter
2 tablespoons all-purpose flour
1 ½ cups whole milk
1 cup chicken broth
½ teaspoon salt
¼ teaspoon ground nutmeg
½ (3 pound) rotisserie chicken, boned and shredded
1 (16 ounce) package potato gnocchi
2 cups baby spinach
¼ cup grated Parmesan cheese
Position a rack in the upper third of the oven; preheat to 425 degrees F (220 degrees C). Heat oil in a large skillet over medium heat. Add mushrooms and season with salt and pepper. Cook until starting to soften, about 4 minutes. Transfer to a plate or bowl. Wipe out skillet. Melt butter in the same skillet. Whisk in flour until a paste forms, about 3 minutes. Whisk in milk and broth until smooth. Simmer, whisking constantly, until thickened, about 5 minutes. Season with 1/2 teaspoon salt and nutmeg. Add mushrooms, chicken, gnocchi, and spinach to the sauce. Stir until gnocchi are coated and spinach wilts, about 3 minutes. Pour mixture into a large baking dish. Sprinkle liberally with Parmesan cheese. Bake in the preheated oven until bubbling, about 20 minutes.