Preheat oven to 325°F. Prepare an 8" x 8" casserole dish with non-stick cooking spray.
Heat the oil in a large pan; add the onions and cook until brown, about 3 minutes. Add the thyme, granulated garlic, garlic salt, black pepper and stock; bring the liquid to a gentle simmer.
Add the frozen veggies and cooked chicken and return the liquid to a simmer. Sprinkle in 1 cup of the potato flakes until the sauce starts to thicken; remove from heat.Add 4 cups of potato flakes to a bowl and slowly add boiling water, per package directions. The finished potatoes should be thick to ensure it sets well. Pour the chicken mixture into the casserole dish and top with mashed potatoes. Use a spatula to make a thin, even layer.
Sprinkle with cheese and bake for 30 minutes until heated through and beautifully browned.
Enjoy this healthy, hearty dish with a glass of Sauvignon Blanc.