Cheesey Chicken Spaghetti

A crowd pleasing casserole. Thank you  Melissa's Southern Style Kitchen

1 lb thin spaghetti
1 medium onion, finely diced
1 celery rib, finely diced
8 oz sliced baby bella mushrooms [or your preference]
olive oil
4 clove garlic, minced
1/2 cup [1 stick] butter or light butter
seasoned salt and black pepper to taste
1/3 cup AP flour
4 cup whole milk
2 tsp dry Italian seasoning
1 tsp seasoned salt
1/4 tsp black pepper
1/3 cup grated Parmesan cheese plus 2 Tbsp
4 oz jar pimentos, well drained
3 cup cooked chicken, roughly chopped
3 cup shredded white sharp cheddar cheese or Italian cheese blend, divided

• Preheat the oven to 350°F.  
• Coat 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.