Brown ground beef, onions and mushrooms over medium heat in a large skillet. Drain fat and season with salt, garlic powder and brown gravy mix. (I use brown gravy mix as a seasoning in this, and the juices will make some gravy. If you want more gravy, add about 1/2 cup of water-I have not tested this!). Stir well and set aside.
Place diced potatoes in an appropriate size saucepan–2 quart size should be fine. Cover with water and add about 1-2 tsp salt.
Boil potatoes until tender. Drain very well.
Mash potatoes with sour cream, until potatoes are not lumpy. Add green onion and bacon–you can reserve a small amount for a garnish, if desired. Stir in cheese–you can reserve about 2 tbs of this for garnish if desired.
Preheat oven to 350 degrees F.
Place reserved ground beef mixture in a 9-inch deep dish pie pan. Distribute evenly across the bottom.
Top with mashed potatoes, covering meat entirely.
Bake at 350 degrees F for about 30 minutes or until potatoes begin to get brown around the edges and peaks.
Remove from oven and garnish with reserved onion, bacon & cheese if you desired to do so.