Preheat oven to 375°F (190°C). Grease a 13x9-inch baking dish.

Par-boil the potatoes: Cook sliced potatoes in boiling salted water for 5 minutes. Drain under cold water and cool immediately. Set aside.

In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add the diced onion and cook for 2 minutes until softened. Add the ground beef, breaking it apart with a wooden spoon while it cooks until browned, about 5-7 minutes. Drain off excess fat.

Stir in the minced garlic and cook for 1 minute until fragrant. Add the tomato paste and stir until completely incorporated. Cook for another minute.

Season the beef mixture with paprika, salt, and pepper. Taste for seasonings and adjust accordingly. Remove from heat and set aside.

Assemble the casserole: Layer 1/3 of the par-boiled potato slices on the bottom of the prepared baking dish. Top the potatoes with half of the ground beef mixture. Spread another 1/3 of potato slices over the beef. Add the remaining beef mixture and top with the final layer of potato slices.

Make the custard mixture: In a separate bowl, whisk the eggs. Then whisk in the milk, salt, and pepper until thoroughly combined. Pour this milk mixture evenly over the layered potatoes and beef.

Bake for 45 to 55 minutes, or until the custard is set and the top is golden brown. Let the casserole rest for 10 minutes before cutting and serving.