Bring a pot of water to boil and boil the diced potatoes until fork tender. While the potatoes are boiling, add the olive oil, onions and mushrooms to a large skillet over medium-high heat. Allow them to cook for 2-3 minutes then add in the ground beef. Cook until you don't see any pink then season with seasoned salt and drain off the excess grease. Set aside for now.

In the same skillet on low heat add in 6 tablespoons of butter. Once its melted sprinkle in the flour. Mix the two together, stirring constantly, allow it to cook for 2-3 minutes. Pour in the beef broth and packet of Lipton Beefy Onion Soup Mix and whisk together. Bring to a simmer while stirring occasionally. Once it reaches a thicker consistency, set some gravy aside for topping the casserole with and then add in the ground beef. Stir everything together and transfer to a 9x13 casserole dish.

Once the potatoes are fork tender, drain the excess water and pour them into a mixing bowl. Add in the butter and allow it to melt. Once melted, add in the milk, season with salt & black pepper to taste and mash together with a potato masher or an electric hand mixer. Once they have a whipped smooth consistency, pour them on top of the meat mixture and smooth off the top.

Top the casserole with the gravy you set to the side, serve & enjoy!