Prepare the Mashed Potatoes: Place the diced potatoes in a large pan filled with water. Add salt to taste.

Bring to a boil and cook until the potatoes are tender.

Drain the potatoes and return them to the pan.

Add milk and butter to the cooked potatoes. Mash until smooth and creamy. Adjust salt to taste.

Prepare the Potato Stuffing: Heat a few tablespoons of olive oil in a skillet over medium heat.

Add diced onion and minced garlic. Sauté until the onion becomes translucent.

Add the ground beef to the skillet. Cook for about 10 minutes, breaking the meat into small pieces as it cooks.

Once the beef is fully cooked, stir in chopped fresh parsley and spring onions. Remove from heat.

Assemble the Gratin: Preheat your oven to 200°C (392°F).

In a glass baking dish, spread a layer of the mashed potatoes.

Sprinkle a generous layer of grated mozzarella over the mashed potatoes.

Evenly spread the cooked ground beef mixture over the cheese.

Add another layer of mashed potatoes over the beef.

Top with another layer of grated mozzarella cheese.

Bake the Gratin: Place the baking dish in the oven and bake for about 40 minutes, or until the top is golden brown and bubbly.

Serving: Let the gratin cool slightly before serving to allow the layers to set, making it easier to cut and serve.