For the sauce:
4 tablespoons butter, divided
8 ounces cremini mushrooms, diced
3 tablespoons all-purpose flour
2 cups chicken broth
1 cup milk
For the casserole:
1 lb spaghetti, broken into 2-inch pieces
3 cups cooked chicken, shredded
4 slices bacon, cooked and chopped
1 small yellow onion, chopped
1/2 green bell pepper, chopped
1 (4-ounce) jar pimentos, drained
1 teaspoon seasoned salt
2 1/2 cups grated cheddar cheese, divided
1 cup chicken broth
Kosher salt and freshly ground black pepper, to taste
Preheat oven to 350°F.
In a large saucepan, melt 1 tablespoon of the butter over medium-high heat. Add mushrooms and cook until soft, 3-5 minutes.
Add remaining butter. Once melted, sprinkle in flour and cook 1 minute more. Whisk in chicken broth and milk gradually until combined. Season with salt and pepper and cook until thickened, about 5 minutes. Remove from heat and set aside.
Meanwhile, in a large pot of salted boiling water, cook the spaghetti according to package directions. Drain and place in a large bowl.
To the cooked spaghetti, add chicken, bacon, onion, bell pepper, pimentos, seasoned salt, and 1 3/4 cup cheese. Pour sauce over and stir to combine.
Transfer to a 9×13-inch baking dish and pour chicken broth over the top. Top with remaining cheddar cheese. Bake until bubbly and cheese has melted, 35-40 minutes. Enjoy!