Bring a large pot of water to a boil. Add potato chunks and cook until tender, about 15-20 minutes.
Drain potatoes and mash with butter, heavy cream and salt.
Spread potatoes out in an even layer in the bottom of a cast iron pan.
Preheat oven to 350° F.
While potatoes are cooking start the beef. Cook the ground beef and diced onion over medium heat, until the onions are soft, breaking up the beef into crumbles as it cooks.
Season beef with mushroom seasoning, paprika, salt and pepper. Stir to combine. Drain off any excess fat.
Spread the seasoned beef over the layer of mashed potatoes in an even layer.
In the same skillet that you cooked the beef, add 2 tablespoons butter and sauté sliced mushrooms until they release their liquid and all the liquid cooks off, about 8-10 minutes.
Season mushrooms with salt and pepper.
Spread sautéed mushrooms over the layer of seasoned beef.
Top everything with grated cheese.
Bake uncovered for 15 to 20 minutes until heated through.
Switch oven setting to broil and continue cooking until cheese begins to brown along the edges and everything is hot and bubbly.