Cook onion in butter until tender. Add soup, water, and cheese; heat until cheese melts. In buttered casserole dish, combine chicken, spaghetti, parsley, pimiento, and mushrooms. Add soup mixture. Top with extra shredded cheese. Bake at 350 F. for 20 minutes or until bubbly.
If mixture seems too dry, add a little milk.
Serving Size: 8