In 3-quart saucepan, cook and drain spaghetti as directed on package; set aside.
In same saucepan, melt butter over medium-high heat. Add onions and mushrooms; cook, stirring frequently, until tender. In small bowl, mix flour, garlic powder, pepper, broth and milk until smooth. Gradually stir into onion mixture, cooking and stirring until bubbly and thickened.
Stir in chicken, pimientos and sherry. Cook, stirring occasionally, until hot. Stir in cheese. Add cooked spaghetti; toss gently. If desired, serve with additional grated Parmesan cheese and chopped fresh parsley.