Preheat oven to 350°F. For the Filling, melt butter in large saucepan on medium heat. Add onion; cook and stir 5 minutes or until tender. Stir in vegetables, chicken, stock, sage and salt. Mix milk and cornstarch in small bowl with wire whisk until smooth. Stir into chicken mixture. Bring to boil and boil 2 minutes or until thickened, stirring frequently. Pour into 9-inch deep dish pie plate or 2-quart baking dish.
For the Dumplings, beat eggs in large bowl. Add remaining ingredients; mix until a soft dough is formed. Drop by heaping tablespoons over chicken mixture.
Bake 25 to 30 minutes or until a toothpick inserted into a dumpling comes out clean. Let stand 5 minutes before serving.