In a large skillet, melt butter and cook onion until softened. Stir in vegetables, chicken, soup, milk, thyme, salt, pepper, and garlic powder. Cook until bubbling and hot, about 5 minutes. Transfer mixture to the prepared baking dish.
2 Tablespoons butter, 1 medium onion, 2 cups frozen mixed vegetables, 3 cups cooked chicken, 10.5 ounces cream of chicken soup, 1 ½ cups milk, 1 teaspoon dried thyme, ½ teaspoon salt, ½ teaspoon black pepper, ½ teaspoon garlic powder