Preheat oven to 300ºF. Lightly spray a 9×13-inch baking dish with cooking spray.: When the oven warms to 300ºF , you'll notice a steady, dry heat that encourages even baking rather than an aggressive roast. The light coating of cooking spray prevents the casserole from sticking and helps the edges brown gently. A hot oven also kickstarts bubbling without overcooking the top. Be careful if you spray too much, as pooled oil can create uneven browning; one thin mist is all you need. A common mistake here is preheating at the wrong temperature, which can lead to underbaked center or over browned edges, so double check your dial.
Cook pasta according to package directions. Drain.: As the linguine simmers, the kitchen fills with an inviting wheat aroma, and you can hear a gentle rolling boil that signals proper heat. Cook to al dente so the strands hold structure when baked, then drain thoroughly so excess water does not dilute the sauce. Rinsing is unnecessary unless the package recommends it, and it can cool the pasta too much. A frequent error is overcooking the noodles, resulting in a mushy texture after baking; test a strand about two minutes before the package time to avoid this.
In a large bowl, combine chopped chicken, soups, sour cream, butter, broth, salt, pepper, garlic powder. Stir in cooked pasta.: As you stir these elements, the mixture will take on a velvety sheen from the sour cream and melted butter , emitting a layered scent of savory soups and roasted chicken . Mixing in the cooked pasta while everything is warm helps the sauce cling to each strand. Balance the seasoning after combining rather than before, because the canned soups already contribute salt. If the mixture seems too thick, add small amounts of chicken broth until it reaches a saucy but not soupy consistency. Avoid mixing too vigorously or the pasta can break down and become gummy.
Pour mixture into prepared baking dish. Top with cheeses and parsley. Cover with aluminum foil.: When you spread the casserole into the greased 9×13-inch baking dish , smooth it evenly so the top bakes uniformly, which produces consistent bubbling across the surface. Sprinkle the grated Parmesan and shredded mozzarella in an even layer for a golden finish, then dust with dried parsley for a hint of color. Covering with foil traps steam, allowing the center to heat through without overbrowning the cheese. One pitfall here is piling on cheese unevenly, which leads to some spots burning while others stay pale, so take a moment to distribute it evenly.
Bake covered for 45 minutes. Remove foil and bake an additional 10 to 15 minutes, until bubbly and cheese is melted.: During this covered phase, the casserole will steam and the sauce will meld with the pasta, producing a unified, creamy texture and a warm, savory aroma. After removing the foil, watch the surface closely as the cheese transitions from soft to golden and bubbly, signaled by small pockets of steam and a gentle crackle at the edges. The visual cue to look for is even browning and sustained bubbling around the sides, which tells you the center is hot and the cheese is set. Avoid leaving it uncovered too long, which can dry out the top, or not long enough, which leaves the cheese only partially melted. If you notice excess liquid pooling after baking, allow it to rest for several minutes; that helps the sauce thicken and settle before serving.