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.