Onions peel, quarter and in strips cut. Knead well the ground pork, minced meat, breadcrumbs, mustard, 1 teaspoon salt and pepper. Cut mozzarella into cubes. Form about 8 meatballs, put one cube of mozzarella in the middle of each and cover well with the minced meat mixture.
Heat the oil in a large ovenproof pan. Fry the meatballs on each side and remove.
Sauté the onions in the frying fat. Add tomato paste and sauté briefly. Dust with flour and sauté while stirring. Deglaze with wine and stock while stirring. Place the meatballs on top. Bake in a preheated oven (electric cooker: 200 °C/ circulating air: 175 °C/ gas: see
manufacturer) cook for about 10 minutes.
Wash, peel and cut the potatoes into pieces. Cook in boiling salted water for about 20 minutes. Mix the remaining mozzarella with Gouda. Spread over the meatballs and cook in the oven at the same temperature for another 15 minutes.
Drain the potatoes and let them evaporate briefly. Add milk and butter and mash to a puree, season to taste with salt and nutmeg. Remove the meatballs from the oven and sprinkle with parsley. Add mashed potatoes.