Saute mushrooms and onion in butter in large saucepan until tender. Add flour, salt, poultry seasoning and pepper; stir well. Cook 1 minute, stirring constantly. Gradually add broth; cook over medium heat, stirring constantly until thickened and bubbly.
Add cream and stir until smooth. Stir in chicken. Place half of spaghetti in a greased shallow 2 quart casserole. Spoon half of chicekn mixture over spaghetti. Repeat layers. Bake, uncovered, at 350* for 20 minutes. Sprinkle with cheddar cheese and bake an additional 5 minutes (or until cheese melts).