Preheat Oven to 350˚F and butter a 9x13 Casserole dish.

Cook pasta in a large pot of salted boiling water (it should taste like ocean water) and cook until barely al dente according to package instructions - do not overcook. Reserve ½ cup of pasta cooking water, then drain the pasta, partially cover, and set aside.

Saute the Veggies: Place a large skillet over medium/high heat. Add oil then add sliced mushrooms and sauté 3 min or until softened. Add diced onions and cook until onions are soft and golden, 5-7 min. Add minced garlic and sauté another 1 min until fragrant. Transfer to the cutting board with chicken.

Make the Sauce: In the same pot, melt butter and whisk in flour until lightly golden (1 ½ min). Slowly whisk in chicken broth, lemon juice, salt, and pepper and whisk until smooth. Add half and half and bring to a simmer. Season the sauce with more salt and pepper to taste.

Combine the Casserole: Into the pasta pot, add the chicken, mushrooms mixture, parsley, and all of your sauce. Stir to combine. You can add ½ cup of reserved pasta water to make it saucier then spread it evenly into your prepared casserole dish. Sprinkle the top generously with mozzarella.

To Bake: Cover with foil and bake at 350˚F for 20 min, then remove the lid and continue baking for 15 min. You can broil for the last 1-2 minutes to get some golden spots on top.