Preheat oven to 350°F. Spray the 9x13 casserole dish with nonstick cooking spray.
Boil the spaghetti in salted water until al dente. Save 1 cup of pasta water. Drain and set aside.
In a large skillet, heat a drizzle of olive oil over medium heat. Add the mushrooms and sun dried tomatoes. Cook until browned and tender (about 5–6 minutes). Remove from heat.
To the sauteed mushrooms, add the cream of chicken soup, chicken stock, cream cheese, butter, garlic powder, and Italian dressing mix. Stir until smooth and creamy.
Stir in the shredded rotisserie chicken, half the shredded cheese and pasta. Toss until combined. Add the pasta water, ¼ to ½ cup. You want the casserole to be nice and creamy.
Spoon the pasta mixture into the prepared casserole dish. Top with the remaining shredded cheese.
Cover the dish with aluminum foil and bake for 25 minutes. Uncover and broil for 2–3 minutes until the top is golden and bubbly.
Notes
This will keep in the refrigerator for up to 3 days, freeze remaining leftovers. (They freeze great!) It reheats best in the oven, warm until hot and bubbly!