Preheat the oven to 350°F. Spritz a 9 x 13 inch baking dish with cooking spray. Set aside.
Cook the spaghetti in salted water per the package directions until al dente. Drain well.
On the stove top in a medium-size pot, cook the diced onion, mushrooms and celery in a couple of drizzles of olive oil. Cook for 5 minutes until the vegetables soften and are beginning to brown. Season with seasoned salt and black pepper to your taste.
Add the minced garlic and cook for 1 minute longer. Remove to a large mixing bowl.
In the same saucepan melt 1 stick of butter. Add the flour stirring constantly over medium-high heat.
Begin to whisk in the milk. Continue to cook, stirring continuously over medium heat until the sauce comes to a boil. Lower the heat and simmer for 3-5 minutes.
Add 2 tsp dry Italian seasoning, 1 tsp seasoned salt, and black pepper. Adjust salt and pepper to your taste, if needed.
Remove from the heat and add 1/3 cup grated Parmesan cheese.
Add the cooked spaghetti, cooked chicken, pimentos and 2 cups of shredded cheese to the large mixing bowl. Pour the sauce over and stir until combined.
Pour into the baking dish and top with the remaining 1 cup of shredded cheese. Sprinkle with 2 Tbsp Parmesan cheese.
Bake for 30 minutes until golden and bubbly.