In a large saucepan, cook the onion, celery, and garlic in hot butter over medium heat for 3 to 5 minutes or until just tender, but not brown. Stir in the chicken stock, parsnips, salt, black pepper, and bay leaf. Bring to boiling; reduce heat. Simmer, covered, for 25 to 30 minutes or until parsnips are very tender. Remove from heat. Cool slightly (do not drain). Remove bay leaf; discard.