Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
Place potatoes in a large pot; cover with water and add 1 teaspoon salt. Bring to a boil, reduce to a simmer, and cook until tender, about 20 minutes. Drain and mash potatoes with a fork or potato masher. Mix in milk, butter, 1 tablespoon Parmesan cheese, 1/2 teaspoon salt, 1/4 teaspoon pepper, and nutmeg.
Heat olive oil in a large skillet over medium heat. Add chopped onion and cook until translucent, about 4 minutes. Add bell pepper and cook until tender, about 3 minutes. Cook ground beef until browned, 10 to 15 minutes. Mix in smoked paprika, 1 teaspoon salt, garlic powder, and 1/2 teaspoon pepper. Simmer until fully cooked and juicy, about 20 minutes. Let it cool lightly, about 5 minutes. Stir in chopped hard-boiled eggs and olives.
Spread half of the mashed potatoes in the prepared baking dish. Layer beef and egg mixture on top, and cover with the remaining mashed potatoes. Insert mozzarella cubes into the mashed potatoes. Sprinkle with 1 tablespoon Parmesan cheese.
Bake in the preheated oven until golden and beef filling starts to bubble, about 45 minutes.