Combine chicken broth and chicken in heavy large pot. Bring to a boil. Reduce heat, cover partially, and simmer until chicken is cooked through, about 20 minutes. Using tongs, transfer chicken to large bowl. Cool chicken and broth slightly. Discard skin and bones from chicken. Cut chicken meat into bite-size pieces and reserve. Spoon fat off top of chicken broth.
Return broth to simmer. Add onion, carrots and celery. Simmer until vegetables soften, about 8 minutes. (Can be prepared 1 day ahead. Cover chicken meat and broth separately and refrigerate. Bring broth to boil before continuing.)
Melt 2 tablespoons butter in heavy large skillet over medium-high heat. Add mushrooms and saute until they begin to brown, about 5 minutes. Stir in lemon juice. Add mushrooms to broth; stir in noodles, parsley and reserved chicken. Simmer until noodles are tender, about 5 minutes. Season soup to taste with salt and pepper.
Recipe courtesy of Sara Moulton