Preheat oven to 375. Coat a 3 quart baking dish with cooking spray. Cook spaghetti in pot of boiling salted water as pkg. directs. Drain.
Saute celery and onion in butter in large pa over medium high heat 3 minutes. Add mushrooms and bell pepper. Cook 3 minutes. Add garlic; cook 1 minute. Stir in flour, then milk, stirring constantly til thickened. Add cheddar, pimentos, Worcestershire, and nutmeg. Stir to melt Cheddar. Stir in sherry and season with salt, black pepper and red pepper flakes.
Add chicken, almonds and spaghetti to the pan. transfer mixture to prepared baking dish. Sprinkle Parmesan over the top and bake tetrazzini til bubbly, 15 to 20 minutes.