In saucepan, cook first 4 ingredients over medium-high,stirring often, until celery softens, 8 to 10 minutes. Stir in flour. Add vegetables, chicken, and chicken stock. Simmer 2 minutes; season. Transfer to 7-by-11-inch baking dish; spread potatoes evenly over mixture. Bake at 425° until hot and bubbling around edges, about 25 minutes.