Heat oven to 375°F. Grease a 9x13-inch baking dish.
Warm gravy with milk in a saucepan over medium heat until smooth and steamy.
Stir in turkey, mixed vegetables, garlic powder, thyme, and black pepper. Simmer 2 minutes.
Spread stuffing in the prepared dish. Drizzle with half the melted butter and press gently into an even layer.
Pour the turkey-vegetable gravy mixture evenly over the stuffing.
Dollop mashed potatoes over the top, then spread to cover. Brush with remaining butter and sprinkle with salt.
Top with cheddar and green onions.
Bake 30–35 minutes until bubbling and golden at the edges. Rest 10 minutes before serving.