Preheat the oven to 375°F.
In a large mixing bowl, combine the cooked chicken, frozen mixed vegetables, chicken broth, 1 cup of milk, garlic powder, onion powder, thyme, salt, and pepper. Stir until well combined. Pour the mixture into a greased 9x13-inch baking dish.
In another bowl, whisk together the flour, baking powder, and 1/2 teaspoon salt. Add the melted butter and 1/2 cup of milk to the dry ingredients, stirring until just combined.
Drop spoonfuls of the biscuit batter over the chicken mixture in the baking dish, covering as much of the surface as possible.
Bake in the preheated oven for 30-35 minutes, or until the biscuit topping is golden brown and cooked through.
Let cool for 5 minutes before serving.