Preheat your oven to 375 degrees F. In a large bowl combine cooked and diced chicken, veggies and chopped onion. Stir in cream of chicken soup, milk, and garlic powder. Mix well.
Pour the mixture into your preferred baking dish. (I used a round 1 ½ qt dish). In a separate bowl, mix together the Bisquick, egg and milk. Pour batter over the chicken vegetable layer. Bake for approximately 30 minutes, or until the top is fully cooked and slightly browned.
Remove from the oven and let cool slightly so it can thicken more in the center before serving.