In a saucepan, bring chicken stock and unsalted butter to a boil. Add the stuffing mix, stir well, and cover. Remove from heat and let stand for 5 minutes. Fluff with a fork before using.

In a 12 inch skillet (preferably oven proof), melt butter over medium heat. Add flour and cook for 1-2 minutes. Gradually whisk in chicken stock and then add the milk, chicken bouillon cube, garlic powder, onion powder, celery salt, salt, and pepper. Continue to cook and stir until the sauce thickens and the bouillon cube is dissolved (1-2 minutes). Remove from heat.

Add the cooked chicken, and frozen vegetables, mix together with the sauce.

Preheat your oven to 375°F (190°C). If your skillet isn’t oven proof, transfer the mixture to a 9x13 inch (23x33 cm) baking pan.

Layer with the fluffed stuffing to cover the creamy chicken mixture.

Bake the casserole uncovered in the preheated oven for 30 minutes, or until it's hot and bubbly and the top is crispy. You can place it under the broiler for a couple of minutes to crisp up the top if needed.

Let the casserole cool for a few minutes, then garnish with fresh chopped parsley before serving.