Spray a 9x13" baking dish with nonstick cooking spray, and set it aside.
Melt a tablespoon of butter in a large skillet set over medium heat. Once melted, add the vegetables, stirring often, and cooking 6-8 minutes- just until they've softened.
Add the pot roast and gravy to the skillet, stirring to evenly incorporate. Continue cooking until the mixture's heated through.
Add the remaining butter to a separate pot along with the hot water, stirring until melted.
Add the milk, potatoes, and both sauce mix pouches, whisking just until evenly incorporated. Remove the skillet from heat, and let it sit for 2 minutes. Pour this mixture into the prepared baking dish.
Spoon the pot roast and veggie mixture into the baking dish, and then very gently stir it just to incorporate.
Bake the casserole at 450° for 35-40 minutes, or just until the potatoes are tender.
Remove the casserole from the oven and let it rest for 5 minutes.
Serve & enjoy!