Preheat oven to 350ºF. Spray a 9×13-inch baking dish with cooking spray.: When your oven reaches 350ºF , you will notice a steady warm draft if you open the door briefly, and that preheating creates even baking so the casserole cooks uniformly. The aroma of the oven warming is subtle but reassuring, and the hot environment encourages the top to bubble and brown without overcooking the interior. I recommend greasing the pan well so the finished bake releases cleanly, avoiding sticky edges. A common mistake is skipping preheat, which can lead to uneven bubbling and longer bake times; always allow the oven to fully warm before inserting the casserole.
Prepare pasta according to package directions. Drain and place in a large bowl.: After cooking the bowtie pasta to al dente, you should feel a slight resistance when biting, not a mushy collapse. Drain thoroughly so excess water does not thin your sauce, and transfer into a roomy bowl to avoid squashy shapes when mixing. The pasta will still be warm, which helps it absorb some sauce flavor. Avoid overcooking, which leads to a gummy texture when baked; test a piece a minute before the package time ends to prevent that.
In a medium saucepan, melt butter over medium-high heat. Whisk in flour and cook for 1 minute. Slowly whisk in milk, salt, pepper, garlic powder, and onion powder. Keep whisking until smooth. Bring mixture to a boil, reduce heat and stir in cheddar cheese until smooth.: As the butter melts and foams, you will smell a light, toasty butter scent; adding the flour and cooking it for a minute removes raw flour flavor and creates a nutty aroma. Gradually whisk in the milk so the mixture stays silky, and keep whisking to prevent lumps. When it reaches a gentle boil you may hear tiny rapid pops and see steam rising, signals the sauce has thickened. Reducing heat before adding the cheddar cheese prevents grainy separation, and stirring until the cheese is fully integrated yields a glossy, velvety sauce. A frequent mistake is adding cheese to boiling liquid, which can make the sauce break; always lower the heat and stir gently.
Pour cheese sauce over pasta. Stir in cooked chicken. Spoon pasta mixture into a lightly greased 9×13-inch pan. Pour spaghetti sauce over pasta and top with mozzarella cheese.: At this stage the kitchen fills with a warm dairy aroma and the sound of sauce coating the pasta . Stirring the shredded cheddar cheese sauce through the warm pasta helps the sauce cling into the pasta creases. Folding in the chopped chicken distributes hearty bites and keeps texture consistent. Spoon the mixture into the prepared pan with care so layers form evenly, then pour the jarred spaghetti sauce over the top in a thin, even layer. Finish with a spread of shredded mozzarella cheese so it melts into a golden top. One pitfall is packing the pan too tightly; allow some room so heat circulates and the top can brown evenly.
Bake uncovered for 35-40 minutes, until bubbly.: As the casserole bakes, you will hear a soft, steady bubbling from the edges and smell roasted tomato and toasted cheese. The top should develop light golden patches and the center should be bubbling through the sauce; a gentle jiggle indicates it is hot and cooked. Baking uncovered encourages a slightly crisp top, while the interior stays moist. If you find the top browning too quickly, tent loosely with foil to prevent burning while allowing the center to finish. Underbaking leaves the center cool and sauce runny, so confirm bubbling before removing the dish.
