Begin by combining the chicken with enough water to cover it in a large skillet; cover and bring to a boil. Reduce the heat, and then simmer for about 20 minutes, or until the chicken is done. Drain the chicken, chop it, and set aside.
Cook the spaghetti according to the package directions, omitting any salt and fat. Drain, and set aside.
In a small saucepan, combine the soup, milk, and seasonings, stirring well to mix.
Preheat your oven to 325 degrees F and then layer the chicken, spaghetti, soup mixture, and mushrooms in a 12x8x2-inch baking dish.
Bake for roughly 30 to 45 minutes, or until thoroughly heated.