Cook pasta according to package directions. Drain; set aside.
Meanwhile, bring water to a boil in Dutch oven over high heat. Add chicken breasts, onions, celery, salt, thyme, bay leaf and pepper. Return to a boil. Reduce heat to low; simmer, uncovered, 35 minutes. Remove chicken; cut into 1/2-inch pieces. Set aside.
Increase heat to high. Return liquid in Dutch oven to a boil. Continue cooking until liquid and vegetables have reduced to 1 cup. Remove from heat; discard bay leaf. Whisk in soup and buttermilk until well blended. Add chicken pieces and pasta; toss to blend. Sprinkle with parsley, if desired.