Place the chicken in a large pot; add water to cover. Add the salt and pepper, celery stalks, and halved onion; bring to a boil. Reduce heat and cook for 30 minutes, or until the chicken is done.
Reserve four cups of the broth, discarding the celery and onion. Cool slightly; cut the chicken into bite-size pieces.
Meanwhile, cook the vermicelli according to the package directions, omitting any oil and salt. Drain and set aside.
In a large pot, melt the margarine over medium heat, and sauté the chopped onion, green pepper, celery, mushrooms, and garlic until tender.
Gradually stir in the flour, mixing for 30 seconds. Gradually add the reserved chicken broth, stirring.
Add the tomatoes and green chilies, cayenne pepper, and Worcestershire sauce. Add the chicken and vermicelli, mixing well.
Divide the mixture into two 2-quart shallow casseroles coated with nonstick cooking spray. Top each casserole with half the Parmesan cheese, parsley, and green onion, and bake for 20 to 30 minutes, or until thoroughly heated. Serve immediately.