Heat oven to 350°.
Spray a 3-quart baking dish with nonstick cooking spray.
Cook the spaghetti in boiling salted water following the package directions.
Rinse, drain and set aside.
In a large sauté pan, melt butter over medium heat.
Add the mushrooms, ham, onion, peas, and parsley.
Saute, stirring, until mushrooms are tender and slightly caramelized (do not brown).
Stir in flour until blended.
Add chicken broth, sherry, nutmeg, salt, and pepper.
Cook, stirring, until thickened.
Stir in fat-free half-and-half and half of the cheese.
Cook until sauce is creamy and all cheese is melted.
Add the pasta and stir to blend.
Pour into the prepared baking dish.
Sprinkle remaining cheese over the top and bake for 25 to 35 minutes, until hot and bubbly.