Season the chicken with seasoned salt and black pepper. Heat olive oil in a large skillet over medium-high heat and cook until golden brown on both sides. Allow the chicken to rest for a few minutes after cooking. Cut it into small bite-sized pieces so it blends evenly throughout the pasta. The browned bits left in the skillet will add amazing flavor to the sauce later, so do not wash the pan.
Bring a large pot of salted water to a boil. Cook the spaghetti until just tender but not overly soft. Drain the pasta and set it aside. Slightly undercooking the pasta helps prevent it from becoming mushy during baking. Many cooks like to break the spaghetti in half before cooking because it makes serving easier later.
Using the same skillet, melt the butter and cook the diced onion until soft and translucent. Add the garlic and cook briefly until fragrant. Sprinkle in the flour, garlic powder, and Italian seasoning. Stir continuously to form a flavorful roux. Slowly whisk in the chicken stock and scrape the skillet to release the flavorful browned bits from cooking the chicken.
Pour in the heavy cream and continue whisking until smooth. The mixture should begin to thicken into a creamy sauce. Add Parmesan, cheddar, and mozzarella cheese. Stir until completely melted and silky. The combination of three cheeses creates a rich texture and balanced flavor throughout the dish.
Add the drained diced tomatoes and green chilies to the sauce. Stir everything together thoroughly. The tomatoes provide freshness while the green chilies contribute a mild kick without overwhelming heat. Taste the sauce and adjust seasoning if desired before combining everything.
Place the cooked spaghetti into a large mixing bowl or leave it in the pot. Add the chicken pieces and creamy sauce. Use tongs to gently toss everything together. Make sure every strand of spaghetti is coated with the sauce. At this stage, the mixture may seem extra creamy, but it will thicken beautifully during baking.
Pour the spaghetti mixture into a greased 9×13-inch baking dish. Spread it evenly using a spoon or spatula. Top the casserole with the remaining cheddar and mozzarella cheese. The extra cheese creates a bubbly, golden topping that makes the dish irresistible.
Bake in a preheated 375°F (190°C) oven until the edges are bubbling and the cheese is golden brown. Remove the dish from the oven and allow it to cool for a few minutes before serving. This brief resting period helps the casserole set and makes serving much easier.