Saute mushrooms in margarine until softened.

In a separate saucepan, mix cold skim milk with cornstarch until smooth.

Add chicken broth, pepper, salt, onion salt, and nutmeg to the saucepan.

Heat the sauce over low heat, stirring constantly, until it thickens.

Simmer the sauce for 5 minutes to ensure it is fully thickened.

Boil egg noodles according to package directions until al dente.

Drain the cooked noodles thoroughly.

In a large bowl, combine the cooked noodles and sauteed mushrooms.

Pour the thickened sauce over the noodle and mushroom mixture.

Add the cooked chicken pieces and mix well to combine.

Transfer the mixture to a baking dish.

Sprinkle grated Parmesan cheese evenly over the top of the casserole.

Bake in a preheated 375°F (190°C) oven for approximately 20 minutes, or until the top is browned and bubbly.

Remove from the oven and let cool slightly before serving.

Serve directly from the casserole dish.