Combine rotisserie chicken and soy sauce together to coat the chicken and marinate in the refrigerator for 30-60 minutes.
Heat oven to 400 degrees. Spray a deep 8X8 baking dish with cooking spray.
Bring 2 quarts of salted water to a boil. Cook spaghetti until al dente (still has a slight bite to it). Reserve 1 cup of pasta water, then drain spaghetti and set aside.
Meanwhile, melt butter in a 12 inch skillet over medium-high heat. Add onion and cook for 2 minutes. Add mushrooms, 1/2 tsp salt and 1/2 tsp pepper and cook until mushrooms release their liquid and start to brown, about 5-6 minutes. Stir in flour and cook for 2 more minutes.
Whisk in broth, sherry, reserved pasta water, scrape up any browned bits, whisk to remove any lumps, and bring to a simmer. Whisk in cream cheese and 6 Tbsp parmesan and simmer until cream cheese melts and slightly thickened, about 1 minute. Stir in rotisserie chicken and frozen peas and simmer 1 minute more. Add in cooked pasta and toss to combine. Taste, and add salt and pepper if needed.
Pour mixture into the baking dish. Toss panko with remaining 2 Tbsp parmesan, and 1 Tbsp olive oil or melted butter. Sprinkle over the top of the pasta. Bake until topping starts to brown and pasta is bubbling throughout, about 15 minutes. Cool slightly before serving.
Enjoy!