Heat your oven to 350°F. Lightly grease a 9x13-inch baking dish—nothing fancy, just enough so things don’t stick.
Boil the linguini in salted water until just al dente, about 8 minutes. Drain and set aside. Don’t let it get too soft.
While the pasta’s going, sauté the onion and mushrooms in butter over medium heat. Five minutes should do it—they’ll get soft and smell amazing.
Toss the cooked pasta, sautéed veggies, chicken, cream of chicken soup, chicken broth, sour cream, garlic powder, salt, and pepper together in a big bowl.
Mix in half the Parmesan and mozzarella. Scoop the whole thing into your prepared dish.
Sprinkle the rest of the mozzarella and Parmesan over the top.
Bake it uncovered for 35–40 minutes, until you see bubbling and a golden top.