The first time I had this recipe was back in high school, and it was from one of the many Weight Watchers cookbooks that always seemed to accumulate in our house. Unfortunately, I can't find that particular version. If anyone has that version, one that uses a bouillon cube and no sherry, I'd really, really appreciate a little note. Oh, yeah... when you make this recipe, remember one thing. Milk expands when heated. I mean really expands. Make sure your casserole dish has enough room at the top to allow the sauce to bubble.
Cook spaghetti as per directions, drain. Melt 1 tablespoon butter in medium pot. Add onion, garlic and mushrooms, cook until tender, about 3 minutes. Remove and set aside. Preheat oven to 400� F. Melt remaining 4 tablespoons butter in pot. Whisk (add and stir together to remove lumps) in flour and cook, stirring constantly, for 2-3 minutes, without browning. Whisk in milk and season to taste with salt and cayenne pepper. Cook until slightly thickened (be patient). Add mushroom mixture, chicken and sherry. In a buttered 2-quart baking dish, arrange a layer of spaghetti, then a layer of mixture, until dish is full. Sprinkle with cheese and paprika. Bake 30 minutes. Let cool about 10 minutes before serving, to allow the sauce to thicken slightly. Serve with garlic bread.