Start by bringing a large pot of salted water to a boil. Add the spaghetti and cook until just al dente—usually about 8 to 9 minutes. You want it slightly firm because it will continue to cook in the oven. Once it’s done, drain it using a colander and set it aside while you prepare the sauce.
Avoid rinsing the pasta unless you're making it ahead of time. Keeping that light starch helps the sauce cling better. If you're cooking it early, a splash of olive oil can keep it from sticking together.
While your spaghetti is boiling or draining, grab a large skillet or saucepan. Over medium heat, combine the diced tomatoes with green chiles (undrained), cream of chicken soup, cubed processed cheese, and chopped onion. Stir often as the cheese melts to prevent it from sticking or burning.
This part takes about 5 to 7 minutes. You’re looking for a smooth, creamy mixture with no visible chunks of cheese. Once it's bubbling slightly and the onions are soft, turn the heat down and get ready to add the chicken.
Add Chicken and Combine Everything
Stir in the cooked chicken, and let it warm through for about 2–3 minutes in the sauce. This step helps the chicken soak up some of the flavors from the tomatoes and cheese. Season with salt and pepper to your liking—just don’t forget that processed cheese already adds salt.
Now it’s time to add the drained spaghetti to the skillet or mix everything together in a large bowl if your pan isn’t big enough. Use tongs or a large spoon to toss it gently until every noodle is coated in that creamy, cheesy goodness.
Preheat your oven to 350°F (175°C). Lightly coat a 9x13-inch baking dish with cooking spray to keep everything from sticking. Pour the spaghetti mixture into the dish and spread it evenly.
Bake in the preheated oven for 25 to 30 minutes, or until the top is bubbling and slightly golden. You can add a bit of shredded cheese on top if you like that golden-brown finish—it’s optional but delicious.
Once it’s baked through, let the dish sit for about 5 minutes before serving. This rest time helps the sauce thicken and keeps everything from falling apart when you slice into it. I usually garnish mine with chopped cilantro or sliced green onions just before serving for a pop of color and freshness.