Preheat oven to 350 degrees F.
Melt 1 tablespoon of butter in a large sauce pan, then add the diced mushrooms and cook for 3-5 minutes over medium-high heat, stirring frequently, until most of the liquid released from the mushrooms cooks off.
Add remaining butter and let melt, then sprinkle with the flour and stir and cook for 30 seconds.
Whisk in the chicken broth and milk, about ½ cup at a time, until combined. Stir in salt and pepper, then simmer for 3-5 minutes until slightly thickened. Remove sauce from heat and set aside.
Fill a large pot with water and bring to a boil. Add the spaghetti and season the water with about 1 tablespoon of salt. I also like to add 3-4 tablespoons of chicken-flavored Better Than Bouillon to the water to flavor the spaghetti noodles while they cook. Cook according to package directions until al dente, then drain.
In a large bowl, combine the cooked spaghetti noodles, chicken, onion, bell pepper, pimentos, 1 ¾ cups cheese, seasoned salt, and cayenne pepper. Pour the sauce over everything and stir to combine. Transfer to a 9x13-inch baking dish.
Pour remaining 1 cup chicken broth over the pasta, then sprinkle with the remaining ¾ cup cheese. Bake for 35 to 40 minutes until the cheese is melted and the casserole is hot and bubbly.