Heat oven to 350ºF. In 3-quart saucepan, melt butter over medium heat. Add onion; cook, stirring frequently, until tender. Stir in vegetables, chicken, chicken broth, salt and thyme; heat to boiling. In small bowl, mix 3/4 cup milk and the cornstarch with wire whisk until smooth; stir into chicken mixture. Heat just to boiling. Pour into ungreased 2-quart casserole In small bowl, stir all topping ingredients except parsley with fork until blended. Drop topping mixture by small spoonfuls over chicken mixture. Sprinkle with parsley. Bake 25 to 30 minutes or until toothpick inserted in center of topping comes out clean.