Preheat the oven to 350°F. Spritz a 13x9 inch baking dish with cooking spray. Set aside.
Cook the spaghetti in salted water per the package directions until al dente. Drain well.
Meanwhile, 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. (Lightly season with salt and pepper to taste.)
To the pot add the minced garlic cooking for 1 minute longer. Remove to a large mixing bowl.
In the same saucepan melt 1/2 cup of butter. Add the flour stirring constantly over medium-high heat until the flour is absorbed.
Begin to gradually whisk in the milk. Continue to cook stirring continuously over medium heat until the sauce comes to a boil. Immediately lower the heat and simmer gently bubbling for 5 minutes.
Add 2 tsp dry Italian seasoning, 1 tsp seasoned salt and 1/2 tsp black pepper. Taste and adjust salt and pepper to your taste.
Remove from the heat and add 1/3 cup grated Parmesan cheese.
To the large mixing bowl add the cooked spaghetti, chicken, pimentos and 2 cups of shredded cheese. Pour the sauce over and stir until combined.
Transfer to the baking dish and top with the remaining 1 cup of shredded cheese. Sprinkle with 2 Tbsp Parmesan cheese.
Bake for 30-40 minutes until golden and bubbly.