Cook the Spaghetti: Fill a large pot with water, add salt, and bring to a boil. Break the spaghetti in half and cook until al dente—just firm to the bite. Drain the noodles and set them aside, but don’t rinse them. The slight starch left on the pasta helps the sauce cling better later on.
Preheat the Oven: Set your oven to 350°F (175°C) while you prep the rest of the ingredients. Grease a 9x13-inch baking dish with nonstick spray or a light coating of butter. This prevents sticking and makes for easy cleanup.
Mix the Filling: In a large mixing bowl, combine the shredded chicken, cooked spaghetti, cream of mushroom soup, cream of chicken soup, diced tomatoes with green chilies, chopped onions, seasoned salt, and 1 ½ cups of the shredded cheddar cheese. Stir well until everything is evenly mixed. Taste the mixture and add salt and pepper to your preference.
Transfer and Top with Cheese: Pour the mixture into your greased baking dish, spreading it out into an even layer. Sprinkle the remaining ½ cup of cheddar cheese evenly over the top. This will create a nice melted layer that turns golden in the oven.
Bake the Casserole: Place the dish in your preheated oven and bake uncovered for about 40 to 45 minutes. The casserole should be hot throughout and bubbling around the edges. If you prefer a less browned top, cover it loosely with foil during the first 25 minutes of baking, then remove the foil for the last 15–20 minutes to allow the cheese to melt and slightly crisp.
Rest and Serve: Once out of the oven, let the casserole rest for about 5 to 10 minutes. This gives the cheese and sauce time to settle, making it easier to serve. Slice or spoon out portions and enjoy while hot. Leftovers store well and make excellent reheated meals the next day.