Start by cutting the previously peeled potatoes into small pieces - cut them in half then chop these halves into dices.
Transfer them to a bowl and add enough water to cover them all. Add the salt and cook for 15 to 20 minutes.
Dice the onion.
Into a skillet, add the onion and stir until golden brown, add the ground beef and stir until it releases some juices.
Season with salt, sweet paprika, black pepper and garlic powder. Mix very well for 1 minute. After that, add the tomato sauce and mix well.
Cover the pan and let it cook for 10 minutes.
Once the potatoes have cooked through, drain all the water and transfer them to a bowl, add butter and parsley, mash them well and mix.
Spread some olive oil on your hands and get a chunk of potato, then round it into a ball. I've made 6 balls out of it.
With your fingertips, press down on the center of each potato ball, making room for the parmesan cheese and for the ground meat, once it's cooked through.
Split the cheese and the ground beef into equal portions and add them to the center of each potato ball.
Grate the mozzarella cheese and sprinkle some of it over each potato ball.
Bake in a preheated oven at 180°C (356°F) for 15 to 20 minutes.
Serve.