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