I love chicken tetrazzini. I just guessed how it was made and it turned out great. Even my picky fiance loved it!
Bring a large pot of salted water to a boil. Stir in the spaghetti; boil until the pasta has cooked through, but is still firm to the bite, about 12 minutes. Drain well.
Preheat oven to 350 degrees F (150 degrees C). Use the butter to generously grease a 9x13 inch baking dish.
Layer baking dish with the cooked spaghetti. Cut chicken breasts in half, and layer on top of spaghetti. Combine the chicken broth, mushroom soup, milk, sour cream, celery, and mushrooms in a large bowl. Season with garlic powder, onion powder, and salt and pepper. Pour the mixture evenly over the chicken and pasta. Top casserole with 1/2 cup bread crumbs.
Bake in preheated oven for 45 minutes, turning dish after 25 minutes to brown evenly.
Per Serving: 495 calories; protein 23.8g; carbohydrates 54.4g; fat 20g; cholesterol 66.1mg; sodium 403.4mg.