Pre-heat oven to 400F.
In a large saucepan, over medium high heat, cook ground been with a dab of butter.
Once the ground beef is fully cooked, add the diced mushrooms and onions to the pan.
Continue cooking until the onions are translucent: drain the liquid from the pan.
Add the gravy mix and cream of mushroom soup to the pan; stir until all of the meat is coated.
Place the meat mixture into an iron skillet or greased 9 x 13-inch baking dish; spread out evenly.
Place the water and butter into a large saucepan and bring to a boil.
Once the liquid is boiling, immediately remove the pan from the heat and add the milk and mashed potato mix; stir gently and then leave to sit for one minute.
After one minute, stir the potatoes again and spread on top of the meat mixture in the baking dish or iron skillet.
Place the iron skillet or baking dish in the oven and bake the casserole until the potatoes are starting to brown around the edges.
Remove from the oven and let rest for a few minutes before slicing and serving.
