1. Preheat the oven to 350 °F (about 175 °C). Grease a 9×13‑inch casserole dish (or equivalent).
2. In a large nonstick or heavy skillet over medium heat, melt the butter. Add the chopped onion and sauté until soft and translucent, about 4 to 5 minutes.
3. Stir in the seasoned salt and garlic powder. Then add the cream of chicken soup, the Rotel tomatoes with green chilies, the cubed pasteurized cheese product, and the milk.
4. Cook, stirring constantly, until the cheese melts fully and the sauce is smooth and creamy. Be patient here — you don’t want lumps, and you want a silky texture.
5. Fold in the chopped chicken. Season with freshly ground black pepper to taste (start conservatively, you can add more later).
6. Next, fold in the cooked spaghetti, ensuring the noodles are well coated in the creamy, spicy sauce.
7. Transfer everything into the prepared casserole dish, spreading the mixture evenly. Cover with aluminum foil and bake for 20 minutes.
8. After 20 minutes, uncover and sprinkle the shredded cheddar cheese evenly on top. Return to the oven (uncovered) and bake an additional 5 minutes, or until the cheese is melted and just starting to lightly brown.
9. Let the casserole rest for 5–10 minutes before serving so it sets slightly (this helps it hold together better when scooping).

Your Spicy Chicken Spaghetti is ready to enjoy!