In 5-quart Dutch oven, melt butter over medium-high heat. Add chicken mixture. Cook 7 to 9 minutes, stirring occasionally, until no longer pink in center. Stir in onion, celery and garlic. Cook 2 to 3 minutes, stirring occasionally, until vegetables have softened. Sprinkle flour over chicken-vegetable mixture, stirring constantly until mixture is bubbly. Slowly stir in chicken stock. Heat to boiling; reduce heat to low. Simmer uncovered 10 minutes, stirring occasionally. Slowly add cheese, 1/2 cup at a time, stirring constantly with whisk until cheese is melted and heated through.