Slice the boiled potatoes into even pieces. In a large skillet, heat olive oil over medium heat. Add butter, allowing it to melt and blend with the oil. Fry the potatoes until golden and crispy. Halfway through, add finely chopped garlic and a generous sprinkle of paprika, salt, and black pepper.
In a mixing bowl, combine the chopped blue onion, canned tuna, diced pickled cucumbers, and dill. Add mayonnaise, salt, and black pepper to the mixture. Stir well until the sauce is smooth.
Once the potatoes are cooked, sprinkle with mozzarella cheese and allow it to melt slightly. Serve the crispy potatoes garnished with fresh parsley and optional Parmesan cheese. Offer the tuna and cucumber sauce on the side for dipping or topping.