Original recipe found here

Serves 4.
Preparation 30 min
Cooking 30 min
Standing 1 h

Ingredients
100g potatoes, peeled
4 tsp olive oil
1 tsp chopped fresh Italian parsley
1/4 onions, finely chopped
1 large egg
400g ground beef
1 1/2 tbsp milk
1/4 tsp allspice powder
1/4 tsp salt, or to taste
ground pepper to taste
Sauce:
1 large egg
2 tsp Italian parsley, fresh, minced
5 tbsp brown sugar
4 tbsp Dijon mustard
4 tsp apple cider vinegar
4 tsp canola oil

Method
1. Peel and boil the potatoes until they are very tender, 20-25 min. Drain them well, then put them in a bowl and mash.
2. Mince the parsley, then add half to the bowl of the potatoes and reserve the rest. Finely chop the onion, then add it to the bowl. Add the egg, ground beef, milk, allspice, salt, and pepper. Using your hands, blend the mixture together until it is just combined well (do not over mix). Form the mixture into 2-3 cm balls and arrange them on a tray. Cover and chill in the refrigerator for at least 1 h, or up to overnight. The purpose is to firm up the meat mixture.
3. Meanwhile, in a small bowl, whisk together all sauce ingredients, until the brown sugar is dissolved and a sauce consistency is obtained. Set aside.
4. Preheat the oven to 175°C. Line a baking sheet with aluminum foil.
5. Heat the olive oil in a large pan over medium heat. Working in batches, add the meatballs and sauté until they are golden on all sides. Transfer the meatballs to the prepared baking sheet and brush them with the sauce.
6. Cover loosely with foil and roast in the middle of the oven 25-30 min. Serve the meatballs with any remaining sauce on the side.