Preheat oven to 350 degrees.
Melt butter in a large nonstick pan. Add onion and cook until soft, about 4 to 5 minutes.
Add seasoned salt, garlic powder, chicken soup, tomatoes, pasteurized cheese product, and milk. Cook, stirring constantly until cheese is melted.
Fold in chicken. Season to taste with pepper.
Fold in spaghetti and pour into a greased 9X13-inch casserole dish. Cover with aluminum foil and bake for 20 minutes.
Uncover dish and sprinkle shredded cheese on top. Return to oven and bake uncovered for 5 minutes.