Preheat your oven to 350˚F (175˚C) and butter a 9x13 casserole dish to prevent sticking.

Cook the pasta in a large pot of salted boiling water until barely al dente. Reserve 1/2 cup of the pasta cooking water, then drain the pasta and set aside.

In a large skillet over medium-high heat, sauté the mushrooms in olive oil for about 3 minutes until softened. Add the chopped onions and cook until golden, about 5-7 minutes. Add the minced garlic and sauté for another minute. Transfer this mixture to a cutting board with the cooked chicken.

In the same pot, melt the butter and whisk in the flour until lightly golden, about 1 1/2 minutes. Slowly whisk in the chicken broth, lemon juice, salt, and pepper until smooth. Add the half & half and bring to a simmer. Adjust seasoning with salt and pepper as needed.

Combine the chicken, mushroom mixture, chopped parsley, and sauce back in the pasta pot. If you prefer a saucier consistency, add the reserved pasta water. Spread the mixture evenly into the prepared casserole dish and top with shredded mozzarella.

Cover with foil and bake at 350˚F (175˚C) for 20 minutes. Then, remove the foil and continue baking for another 15 minutes. For a golden top, broil for the last 1-2 minutes, keeping a close eye to avoid burning.