In 5-quart Dutch oven, heat butter over medium heat. Cook onion in butter 6 minutes, stirring occasionally, until softened. Add flour; cook and stir 2 minutes. Gradually add stock, stirring until blended. Add chicken, carrots, celery, whipping cream, 2 teaspoons thyme, the salt and pepper. Heat to boiling over medium-high heat. Reduce heat to medium; simmer 12 to 15 minutes or until vegetables are crisp-tender.