Cook the Pasta: Begin by cooking the spaghetti or egg noodles according to the package instructions, boiling them until they are al dente. Once cooked, drain the pasta and set it aside to cool slightly.
Sauté the Vegetables: In a large skillet over medium heat, melt 2 tablespoons of butter. Add the diced onion and sliced mushrooms to the skillet. Sauté for about 5 minutes, or until the onions are translucent and the mushrooms are tender.
Add Garlic and Create a Roux: Stir in the minced garlic and cook for an additional minute, allowing it to become fragrant. Next, sprinkle in the flour, mixing it well with the vegetables to form a roux.
Make the Sauce: Gradually pour in the chicken broth and cream while whisking continuously. Keep whisking until the mixture is smooth and starts to thicken, which should take about 4-5 minutes.
Combine the Chicken and Peas: Add the shredded chicken, frozen peas, Italian seasoning, salt, and pepper to the skillet. Stir everything together until thoroughly combined and heated through.
Preheat the Oven: As you prepare the casserole, preheat your oven to 350°F (175°C).
Combine Pasta and Chicken Mixture: In a large mixing bowl, combine the cooked noodles with the chicken mixture. Stir in half of both the mozzarella and Parmesan cheeses, ensuring an even distribution throughout the mixture.
Transfer to Baking Dish: Pour the entire mixture into a greased 9x13 inch baking dish. Evenly spread the remaining mozzarella and Parmesan cheeses over the top.
Add Butter: Dot the surface with the remaining 2 tablespoons of butter, cutting it into small pieces and distributing them over the cheese.
Bake: Cover the dish tightly with foil, then bake in the preheated oven for 25 minutes. After that time, remove the foil and continue to bake for an additional 10-15 minutes, or until the cheese is bubbling and turns a delightful golden brown.
Serve and Garnish: Once baked, allow the dish to cool for a few minutes before serving. Garnish with freshly chopped parsley for a pop of color and flavor.
Notes
Serve directly from the baking dish for a homey feel, or spoon into individual bowls.