In a large mixing bowl, mash the potatoes and mix in cream cheese, nutmeg, and salt.
Heat a thin layer of oil in a cast iron skillet (or frying pan if you plan on baking in a casserole dish), brown the ground beef and add the onions, garlic, salt, pepper, and rosemary.
When the ground beef mixture is cooked through, add the tomato sauce and chunks of tomato.
If cooking with a cast iron skillet, you can simply add the next ingredients to the pan and bake. If not, transfer the ground beef to a casserole dish first. Then add the parmesan to the beef mixture, layer on the mashed potatoes, and finally, cover with a generous helping of mozzarella. Bake at 375°F for 30 minutes.