Bring stock to a boil in a large saucepan over medium-high heat. Add chicken; reduce heat, and simmer 18 to 20 minutes or until chicken is done. Remove chicken from pan; reserve stock. When cool enough to handle, shred with 2 forks.
Heat a large skillet over medium-high heat. Add oil; swirl to coat. Add onion, garlic, 1/2 teaspoon salt, thyme, pepper, and mushrooms; sauté 8 minutes. Stir in spinach, 1 package at a time, cooking 2 minutes after each addition or until spinach wilts before adding more. Remove pan from heat; stir in chicken.
Preheat oven to 375°.
Coat 2 (8-inch) square baking dishes with cooking spray. Combine milk, flour, and nutmeg, stirring with a whisk until smooth. Bring reserved stock to a boil over medium-high heat. Gradually add milk mixture to stock mixture, stirring constantly with a whisk. Stir in remaining 1 teaspoon salt. Cook 5 minutes or until thickened, stirring occasionally. Remove pan from heat; stir in Parmigiano-Reggiano. Pour 1/2 cup sauce into bottom of each baking dish. Top each with 2 noodles, 1 1/2 cups chicken mixture, 3/4 cup sauce, and 1/4 cup mozzarella, making sure noodles are covered with sauce. Repeat layers once with noodles, chicken mixture, and sauce. Top dishes evenly with remaining 4 noodles, remaining 1/2 cup sauce, and remaining 3/4 cup mozzarella.
Cover dishes tightly with foil coated with cooking spray. Bake at 375° for 25 minutes. Uncover and bake at 375° for 10 to 15 minutes or until browned and bubbly, or follow freezing instructions. Let stand 5 minutes before serving.
How-to Freeze: Cool pan completely. Cover with foil; freeze up to 2 months. Thaw: In metal pan: Thaw overnight in refrigerator. In glass or ceramic: Uncover and microwave at HIGH 15 minutes or until thawed. Reheat: Cover; bake at 375° for 25 minutes. Uncover and bake 5 minutes.
This was delicious! I did add some chix broth right before I put it in the oven as we prefer saucy over sturdy and used a regular 9x12 aluminum pan...didn't have anything to freeze but we got about 10 servings out of it. Definitely making it again!
Loved the idea of having a second dish in the freezer. I took the metal dish out of the freezer the night before and it wasn't quite thawed 24 hours later. I did find the dish took about 45 minutes to bake and "bubble" rather than the 25 minutes. This is definitely on our list to make again.
I've made this recipe several times and finally figured out to cover the chicken while it boils, otherwise you will not have enough liquid to make enough sauce for two dishes. This is a delicious recipe. Kids and hubby love it too!