Preheat your oven to 350˚F. Grease a 9x13-inch baking dish with butter or cooking spray.

Bring a large pot of heavily salted water to a boil. Cook the pasta until it is barely al dente. Reserve 1/2 cup of the pasta cooking water, then drain the pasta and set it aside.

Heat olive oil in a large skillet over medium-high heat. Add the sliced mushrooms and saute for about 3 minutes until softened. Add the chopped onions and continue cooking until they are soft and golden (about 5-7 minutes). Stir in the minced garlic and cook for another 1 minute until fragrant. Transfer the mixture to a plate.

In the same skillet or a pot, melt the butter. Whisk in the flour and cook for about 1.5 minutes until lightly golden and nutty-smelling. Slowly whisk in the chicken broth, lemon juice, salt, and pepper. Continue whisking until the mixture is smooth. Pour in the half and half and bring the sauce to a gentle simmer. Taste and adjust seasoning if necessary.

In the large pot used for the pasta, combine the cooked noodles, shredded chicken, mushroom mixture, chopped parsley, and the creamy sauce. Stir well to coat everything evenly. If the mixture seems too thick, add a bit of the reserved pasta water to reach your desired creaminess.

Spread the mixture into the prepared 9x13 dish. Sprinkle the shredded mozzarella cheese evenly over the top. Cover the dish with foil and bake for 20 minutes.

Remove the foil and bake for an additional 15 minutes. For a golden, bubbly top, you can broil the casserole for the last 1-2 minutes. Garnish with extra parsley and serve warm.