Preheat oven to 350 F.
Grease a 2 1/2- to 3-quart baking dish.
In large skillet or saucepan over low heat, melt butter; sauté mushrooms until tender. Blend in flour, stirring until smooth and bubbly. Gradually add chicken broth, milk, pimiento, and salt and pepper; stir constantly until sauce is thickened.
Stir in chicken, peas, and 1/2 cup of Parmesan cheese, and the cooked noodles. Spoon the mixture into the prepared baking dish.
Sprinkle the remaining 1/4 cup of Parmesan cheese over top.
Bake the casserole for about 25 minutes, until hot and bubbly.