Spray a 9×13-inch pan with cooking spray.: When you prepare the pan, you create a nonstick surface that makes serving and cleanup far easier. I like to evenly coat the bottom and sides so the bubbling casserole releases cleanly. You should hear a faint sputter when the spray hits a warm pan, though here it is simply an even mist on a cool dish. A common mistake is skimping on the spray, which can cause browned cheese or stuck edges, so be generous and wipe the rim free of excess to avoid burning.
In a large bowl combine cooked chicken, cream of chicken soup, spinach, milk, chicken broth, garlic powder, onion powder, 1 cup Monterey Jack cheese, and half of the French fried onions. Stir in uncooked pasta.: As you fold these components, notice the mixture thicken and become glossy from the soup and dairy. The shredded chicken pieces will look moister as they soak up the sauce, and the flecks of green from the spinach will thread through the pasta. Stir until the pasta is evenly coated, and the mixture holds together without being soupy. The aroma of the powdered seasonings will rise gently when stirred, hinting at the savory profile to come. Avoid overmixing vigorously, which can break up the chicken and make the texture less satisfying.
Pour pasta mixture into prepared pan. Cover and refrigerate overnight.: Pouring the mixture should feel satisfying as it fills the pan in a level layer, and the sauce settles around the pasta shapes. Cover tightly to prevent absorption of fridge smells and to allow the pasta to slowly hydrate. Overnight chilling lets the dried pasta swell and infuse with savory flavors, which results in a creamier casserole after baking. A typical oversight is leaving the dish uncovered, leading to a dry skin on top, so be sure the cover seals well.
Remove casserole from refrigerator and uncover.: When you take the pan from the fridge, the surface will look set and slightly firmer from the chilled dairy. Leave it uncovered for a few minutes while you preheat the oven so it comes closer to room temperature, which promotes even baking. You might notice some liquids have pooled; that is normal, and the oven will incorporate them into a cohesive sauce. Don’t skip the brief rest, as a very cold center can lengthen bake time unevenly.
Preheat oven to 350ºF.: Setting the oven to 350ºF ensures a steady, even heat so the pasta finishes cooking without the top burning. The moderate temperature allows the cheese to melt and the sauce to thicken gradually. When preheating, I give my oven at least ten minutes to reach temperature so the casserole starts cooking immediately and predictably. A common misstep is underestimating preheat time, which can make the casserole overbake while the center remains undercooked.
Top the casserole with the remaining 1 cup of Monterey Jack cheese and the remaining half of the fried onions. Bake, uncovered, for 1 hour.: As the casserole bakes, the top will transform from pale to a bubbling, golden surface, with the cheese stretching and browning slightly around edges. The scent will shift to toasty, cheesy notes, and the fried onions will crisp and toast into a nutty, oniony crown. During the hour, check visually for bubbling at the center and a deeply warmed interior. Avoid opening the oven door repeatedly, because temperature swings can affect the final texture. If the top browns too fast, tent loosely with foil for the remaining time, but expect a satisfying sizzle when you remove it.